Merge branch 'info-native'

1 files changed, 509 insertions, 237 deletions

diff --git a/emms-info-native.el b/emms-info-native.el
index a51dad4..f31475f 100644
--- a/emms-info-native.el
+++ b/emms-info-native.el
@@ -1,6 +1,6 @@
-;;; emms-info-native.el --- Native Emacs Lisp info method for EMMS
+;;; emms-info-native.el --- Native Emacs Lisp info method for EMMS -*- lexical-binding: t; -*-
 
-;; Copyright (C) 2020 Free Software Foundation, Inc.
+;; Copyright (C) 2020-2021 Free Software Foundation, Inc.
 
 ;; Author: Petteri Hintsanen <petterih@iki.fi>
 
@@ -34,17 +34,22 @@
 ;; The following file formats are supported:
 ;;
 ;; - Vorbis: Ogg Vorbis I Profile, filename extension ‘.ogg’,
-;;   elemetary streams only.  Based on xiph.org’s Vorbis I
+;;   elementary streams only.  Based on xiph.org’s Vorbis I
 ;;   specification, see URL
 ;;   ‘https://xiph.org/vorbis/doc/Vorbis_I_spec.html’.
 ;;
-;; - Opus: Ogg Opus profile, filename extesion ‘.opus’, elementary
+;; - Opus: Ogg Opus profile, filename extension ‘.opus’, elementary
 ;;   streams only.  Based on RFC 7845, see URL
 ;;   ‘https://tools.ietf.org/html/rfc7845.html’.
 ;;
-;; - FLAC: FLAC streams in native encapsulation format, filename
-;;   extesion ‘.flac’.  Based on xiph.org’s FLAC format specification,
-;;   see URL ‘https://xiph.org/flac/format.html’.
+;; - FLAC streams in native encapsulation format, filename extension
+;;   ‘.flac’.  Based on xiph.org’s FLAC format specification, see URL
+;;   ‘https://xiph.org/flac/format.html’.
+;;
+;; - MP3 files with extension ‘.mp3’ and id3v2 tags.  All id3v2
+;;   versions should work, but many features like CRC, compression and
+;;   encryption are not supported.  Based on id3v2 Informal Standards,
+;;   see URL ‘https://id3.org’.
 ;;
 ;; Format detection is based solely on filename extension, which is
 ;; matched case-insensitively.
@@ -52,14 +57,16 @@
 ;;; Code:
 
 (require 'bindat)
+(require 'cl-lib)
+(require 'emms-info)
 
-(defconst emms-info-native--max-peek-size (* 512 1024)
+(defconst emms-info-native--max-peek-size (* 2048 1024)
   "Maximum buffer size for metadata decoding.
 Functions called by ‘emms-info-native’ read certain amounts of
-data into a temporary buffer while attempting to read metadata
-information.  This variable controls the maximum size of that
-buffer: if more than ‘emms-info-native--max-peek-size’ bytes are
-needed, an error is signaled.
+data into a temporary buffer while decoding metadata.  This
+variable controls the maximum size of that buffer: if more than
+‘emms-info-native--max-peek-size’ bytes are needed, an error is
+signaled.
 
 Technically metadata blocks can have almost arbitrary lengths,
 but in practice processing must be constrained to prevent memory
@@ -72,18 +79,17 @@ exhaustion in case of garbled or malicious inputs.")
   "Ogg format magic capture pattern ‘OggS’.")
 
 (defconst emms-info-native--ogg-page-size 65307
-  "Maximum size for a single Ogg container page.
-Ogg files are read in chunks of this size during decoding.")
+  "Maximum size for a single Ogg container page.")
 
 (defconst emms-info-native--ogg-page-bindat-spec
   '((capture-pattern vec 4)
-    (eval (when (not (equal last emms-info-native--ogg-magic-array))
+    (eval (unless (equal last emms-info-native--ogg-magic-array)
             (error "Ogg framing mismatch: expected ‘%s’, got ‘%s’"
                    emms-info-native--ogg-magic-array
                    last)))
     (stream-structure-version u8)
-    (eval (when (not (= last 0))
-            (error ("Ogg stream structure version mismatch: expected 0, got %s")
+    (eval (unless (= last 0)
+            (error ("Ogg version mismatch: expected 0, got %s")
                    last)))
     (header-type-flag u8)
     (granule-position vec 8)
@@ -93,12 +99,25 @@ Ogg files are read in chunks of this size during decoding.")
     (page-segments u8)
     (segment-table vec (page-segments))
     (payload vec (eval (seq-reduce #'+ last 0))))
-  "Ogg page structure specification.
-Framing and stream structure versions are verified, otherwise the
-data is assumed to be valid.")
+  "Ogg page structure specification.")
+
+(defun emms-info-native--decode-ogg-comments (filename stream-type)
+  "Read and decode comments from Ogg file FILENAME.
+The file is assumed to contain a single stream of type
+STREAM-TYPE, which must either ‘vorbis’ or ‘opus’.
 
-(defun emms-info-native--decode-ogg (filename packets)
-  "Decode at least PACKETS number of packets from Ogg file FILENAME.
+Return comments in a list of (FIELD . VALUE) cons cells.  See
+‘emms-info-native--split-vorbis-comment’ for details."
+  (let* ((packets (emms-info-native--decode-ogg-packets filename 2))
+         (headers (emms-info-native--decode-ogg-headers packets
+                                                        stream-type))
+         (comments (bindat-get-field headers
+                                     'comment-header
+                                     'user-comments)))
+    (emms-info-native--extract-vorbis-comments comments)))
+
+(defun emms-info-native--decode-ogg-packets (filename packets)
+  "Read and decode packets from Ogg file FILENAME.
 Read in data from the start of FILENAME, remove Ogg packet
 frames, and concatenate payloads until at least PACKETS number of
 packets have been decoded.  Return the decoded packets in a
@@ -112,56 +131,68 @@ Only elementary streams are supported, that is, FILENAME should
 contain only a single logical stream.  Note that this assumption
 is not verified: with non-elementary streams packets from
 different streams will be mixed together without an error."
-  (with-temp-buffer
-    (set-buffer-multibyte nil)
-    (let ((npackets 0)
-          (offset 0)
-          (stream (vector))
-          page)
-      (while (< npackets packets)
-        (insert-file-contents-literally filename
-                                        nil
-                                        offset
-                                        (+ offset
-                                           emms-info-native--ogg-page-size)
-                                        t)
-        (setq page
-              (bindat-unpack emms-info-native--ogg-page-bindat-spec
-                             (buffer-string)))
-        (setq offset
-              (+ offset
-                 (bindat-length emms-info-native--ogg-page-bindat-spec page)))
-        (setq stream (vconcat stream (bindat-get-field page 'payload)))
+  (let ((num-packets 0)
+        (offset 0)
+        (stream (vector)))
+    (while (< num-packets packets)
+      (let ((page (emms-info-native--decode-ogg-page filename
+                                                     offset)))
+        (cl-incf num-packets (or (plist-get page :num-packets) 0))
+        (cl-incf offset (plist-get page :num-bytes))
+        (setq stream (vconcat stream (plist-get page :stream)))
         (when (> (length stream) emms-info-native--max-peek-size)
-          (error "Ogg payload is too large"))
-        ;; Look for packet boundaries: every element that is less than 255
-        ;; in the segment table represents a packet boundary.
-        (setq npackets
-              (+ (length (seq-filter (lambda (elt) (< elt 255))
-                                     (bindat-get-field page 'segment-table)))
-                 npackets)))
-      stream)))
-
-(defun emms-info-native--ogg-decode-comments (filename stream-type)
-  "Decode comment header from Ogg file FILENAME.
-The file is assumed to contain a single stream of type
-STREAM-TYPE, which must either ‘vorbis’ or ‘opus’.
+          (error "Ogg payload is too large"))))
+    stream))
 
-Return a list of comments.  Depending on STREAM-TYPE, its
-elements are either of type
-‘emms-info-native--vorbis-comment-header-bindat-spec’ or
-‘emms-info-native--opus-comment-header-bindat-spec’."
-  (let ((packets (emms-info-native--decode-ogg filename 2))
-        stream)
-    (setq stream
-          (cond ((eq stream-type 'vorbis)
-                 (bindat-unpack emms-info-native--vorbis-headers-bindat-spec
-                                packets))
-                ((eq stream-type 'opus)
-                 (bindat-unpack emms-info-native--opus-headers-bindat-spec
-                                packets))
-                (t (error "Unknown stream type %s" stream-type))))
-    (bindat-get-field stream 'comment-header 'user-comment)))
+(defun emms-info-native--decode-ogg-page (filename offset)
+  "Read and decode a single Ogg page from FILENAME.
+Starting reading data from byte offset OFFSET.
+
+Return a plist (:num-packets N :num-bytes B :stream S), where N
+is the number of packets in the page, B is the size of the page
+in bytes, and S is the unframed logical bitstream in a vector.
+Note that N can be zero."
+  (with-temp-buffer
+    (set-buffer-multibyte nil)
+    (insert-file-contents-literally filename
+                                    nil
+                                    offset
+                                    (+ offset
+                                       emms-info-native--ogg-page-size))
+    (let* ((page (bindat-unpack emms-info-native--ogg-page-bindat-spec
+                                (buffer-string)))
+           (num-packets (emms-info-native--num-of-packets page))
+           (num-bytes (bindat-length emms-info-native--ogg-page-bindat-spec
+                                     page))
+           (stream (bindat-get-field page 'payload)))
+      (list :num-packets num-packets
+            :num-bytes num-bytes
+            :stream stream))))
+
+(defun emms-info-native--num-of-packets (page)
+  "Return the number of packets in Ogg page PAGE.
+PAGE must correspond to
+‘emms-info-native--ogg-page-bindat-spec’."
+  ;; Every element that is less than 255 in the segment table
+  ;; represents a packet boundary.
+  (length (seq-filter (lambda (elt) (< elt 255))
+                      (bindat-get-field page 'segment-table))))
+
+(defun emms-info-native--decode-ogg-headers (packets stream-type)
+  "Decode first two stream headers from PACKETS for STREAM-TYPE.
+STREAM-TYPE must be either ‘vorbis’ or ‘opus’.
+
+Return a structure that corresponds to either
+‘emms-info-native--opus-headers-bindat-spec’ or
+‘emms-info-native--vorbis-headers-bindat-spec’."
+  (cond ((eq stream-type 'vorbis)
+         (bindat-unpack emms-info-native--vorbis-headers-bindat-spec
+                        packets))
+        ((eq stream-type 'opus)
+         (let (emms-info-native--opus-channel-count)
+           (bindat-unpack emms-info-native--opus-headers-bindat-spec
+                          packets)))
+        (t (error "Unknown stream type %s" stream-type))))
 
 ;;;; Vorbis code
 
@@ -192,23 +223,53 @@ exhaustion in case of garbled or malicious inputs.
 This limit is used with Opus and FLAC streams as well, since
 their comments have almost the same format as Vorbis.")
 
+(defconst emms-info-native--accepted-vorbis-fields
+  '("album"
+    "albumartist"
+    "albumartistsort"
+    "albumsort"
+    "artist"
+    "artistsort"
+    "composer"
+    "composersort"
+    "date"
+    "discnumber"
+    "genre"
+    "label"
+    "originaldate"
+    "originalyear"
+    "performer"
+    "title"
+    "titlesort"
+    "tracknumber"
+    "year")
+  "Emms info fields that are extracted from Vorbis comments.")
+
 (defconst emms-info-native--vorbis-magic-array
   [118 111 114 98 105 115]
   "Header packet magic pattern ‘vorbis’.")
 
+(defconst emms-info-native--vorbis-headers-bindat-spec
+  '((identification-header struct emms-info-native--vorbis-identification-header-bindat-spec)
+    (comment-header struct emms-info-native--vorbis-comment-header-bindat-spec))
+  "Specification for first two Vorbis header packets.
+They are always an identification header followed by a comment
+header.")
+
 (defconst emms-info-native--vorbis-identification-header-bindat-spec
   '((packet-type u8)
-    (eval (when (not (= last 1))
-            (error "Vorbis identification header type mismatch: expected 1, got %s"
+    (eval (unless (= last 1)
+            (error "Vorbis header type mismatch: expected 1, got %s"
                    last)))
     (vorbis vec 6)
-    (eval (when (not (equal last emms-info-native--vorbis-magic-array))
+    (eval (unless (equal last emms-info-native--vorbis-magic-array)
             (error "Vorbis framing mismatch: expected ‘%s’, got ‘%s’"
                    emms-info-native--vorbis-magic-array
                    last)))
     (vorbis-version u32r)
-    (eval (when (not (= last 0))
-            (error "Vorbis version mismatch: expected 0, got %s" last)))
+    (eval (unless (= last 0)
+            (error "Vorbis version mismatch: expected 0, got %s"
+                   last)))
     (audio-channels u8)
     (audio-sample-rate u32r)
     (bitrate-maximum u32r)
@@ -217,18 +278,17 @@ their comments have almost the same format as Vorbis.")
     (blocksize u8)
     (framing-flag u8)
     (eval (unless (= last 1))
-          (error "Vorbis framing bit mismatch: expected 1, got %s" last)))
-  "Vorbis identification header specification.
-Identification, framing and version data are verified, otherwise
-the data is assumed to be valid.")
+          (error "Vorbis framing bit mismatch: expected 1, got %s"
+                 last)))
+  "Vorbis identification header specification.")
 
 (defconst emms-info-native--vorbis-comment-header-bindat-spec
   '((packet-type u8)
-    (eval (when (not (= last 3))
-            (error "Vorbis comment header type mismatch: expected 3, got %s"
+    (eval (unless (= last 3)
+            (error "Vorbis header type mismatch: expected 3, got %s"
                    last)))
     (vorbis vec 6)
-    (eval (when (not (equal last emms-info-native--vorbis-magic-array))
+    (eval (unless (equal last emms-info-native--vorbis-magic-array)
             (error "Vorbis framing mismatch: expected ‘%s’, got ‘%s’"
                    emms-info-native--vorbis-magic-array
                    last)))
@@ -236,35 +296,46 @@ the data is assumed to be valid.")
     (eval (when (> last emms-info-native--max-vorbis-vendor-length)
             (error "Vorbis vendor length %s is too long" last)))
     (vendor-string vec (vendor-length))
-    (user-comment-list-length u32r)
+    (user-comments-list-length u32r)
     (eval (when (> last emms-info-native--max-num-vorbis-comments)
-            (error "Vorbis user comment list length %s is too long" last)))
-    (user-comment repeat
-                  (user-comment-list-length)
-                  (struct emms-info-native--vorbis-comment-field-bindat-spec))
+            (error "Vorbis user comment list length %s is too long"
+                   last)))
+    (user-comments repeat
+                   (user-comments-list-length)
+                   (struct emms-info-native--vorbis-comment-field-bindat-spec))
     (framing-bit u8)
     (eval (unless (= last 1))
-          (error "Vorbis framing bit mismatch: expected 1, got %s" last)))
-  "Vorbis comment header specification.
-Header type and framing data are verified.  Too long vendor
-string and comment list will also trigger an error.")
+          (error "Vorbis framing bit mismatch: expected 1, got %s"
+                 last)))
+  "Vorbis comment header specification.")
 
 (defconst emms-info-native--vorbis-comment-field-bindat-spec
   '((length u32r)
     (eval (when (> last emms-info-native--max-vorbis-comment-size)
-            (error "Vorbis comment is too long, length %s" last)))
+            (error "Vorbis comment length %s is too long" last)))
     (user-comment vec (length)))
-  "Vorbis comment field specification.
-Too long comment will trigger an error.
-
-This field is used in Opus and FLAC comment structures as well.")
-
-(defconst emms-info-native--vorbis-headers-bindat-spec
-  '((identification-header struct emms-info-native--vorbis-identification-header-bindat-spec)
-    (comment-header struct emms-info-native--vorbis-comment-header-bindat-spec))
-  "Specification for two first Vorbis header packets.
-They are always an identification header followed by a comment
-header.")
+  "Vorbis comment field specification.")
+
+(defun emms-info-native--extract-vorbis-comments (user-comments)
+  "Return a decoded list of comments from USER-COMMENTS.
+USER-COMMENTS should be a list of Vorbis comments according to
+‘user-comments’ field in
+‘emms-info-native--vorbis-comment-header-bindat-spec’,
+‘emms-info-native--opus-comment-header-bindat-spec’ or
+‘emms-info-native--flac-comment-block-bindat-spec’.
+
+Return comments in a list of (FIELD . VALUE) cons cells.  Only
+FIELDs that are listed in
+‘emms-info-native--accepted-vorbis-fields’ are returned."
+  (let (comments)
+    (dolist (user-comment user-comments)
+      (let* ((comment (cdr (assoc 'user-comment user-comment)))
+             (pair (emms-info-native--split-vorbis-comment comment)))
+        (push pair comments)))
+    (seq-filter (lambda (elt)
+                  (member (car elt)
+                          emms-info-native--accepted-vorbis-fields))
+                comments)))
 
 (defun emms-info-native--split-vorbis-comment (comment)
   "Split Vorbis comment to a field-value pair.
@@ -273,18 +344,23 @@ case-insensitive field name with a restricted set of ASCII
 characters.  VALUE is an arbitrary UTF-8 encoded octet stream.
 
 Return a cons cell (FIELD . VALUE), where FIELD is converted to
-upper case and VALUE is the decoded value."
+lower case and VALUE is the decoded value."
   (let ((comment-string (decode-coding-string (mapconcat
                                                #'byte-to-string
                                                comment
-                                               "")
+                                               nil)
                                               'utf-8)))
     (when (string-match "^\\(.+?\\)=\\(.+?\\)$" comment-string)
-      (cons (upcase (match-string 1 comment-string))
+      (cons (downcase (match-string 1 comment-string))
             (match-string 2 comment-string)))))
 
 ;;;; Opus code
 
+(defvar emms-info-native--opus-channel-count 0
+  "Last decoded Opus channel count.
+This is a kludge; it is needed because bindat spec cannot refer
+outside itself.")
+
 (defconst emms-info-native--opus-head-magic-array
   [79 112 117 115 72 101 97 100]
   "Opus identification header magic pattern ‘OpusHead’.")
@@ -293,35 +369,43 @@ upper case and VALUE is the decoded value."
   [79 112 117 115 84 97 103 115]
   "Opus comment header magic pattern ‘OpusTags’.")
 
-(defconst emms-info-native--opus-channel-mapping-table
-  '((stream-count u8)
-    (coupled-count u8)
-    (channel-mapping vec (channel-count)))
-  "Opus channel mapping table specification.")
+(defconst emms-info-native--opus-headers-bindat-spec
+  '((identification-header struct emms-info-native--opus-identification-header-bindat-spec)
+    (comment-header struct emms-info-native--opus-comment-header-bindat-spec))
+  "Specification for two first Opus header packets.
+They are always an identification header followed by a comment
+header.")
 
 (defconst emms-info-native--opus-identification-header-bindat-spec
   '((opus-head vec 8)
-    (eval (when (not (equal last emms-info-native--opus-head-magic-array))
+    (eval (unless (equal last emms-info-native--opus-head-magic-array)
             (error "Opus framing mismatch: expected ‘%s’, got ‘%s’"
                    emms-info-native--opus-head-magic-array
                    last)))
     (opus-version u8)
-    (eval (when (not (< last 16))
-            (error "Opus version mismatch: expected less than 16, got %s"
+    (eval (unless (< last 16)
+            (error "Opus version mismatch: expected < 16, got %s"
                    last)))
     (channel-count u8)
+    (eval (setq emms-info-native--opus-channel-count last))
     (pre-skip u16r)
     (sample-rate u32r)
     (output-gain u16r)
     (channel-mapping-family u8)
-    (eval (> last 0) (struct opus-channel-mapping-table)))
-  "Opus identification header specification.
-Framing and version data are verified, otherwise the data is
-assumed to be valid.")
+    (union (channel-mapping-family)
+           (0 nil)
+           (t (struct emms-info-native--opus-channel-mapping-table))))
+  "Opus identification header specification.")
+
+(defconst emms-info-native--opus-channel-mapping-table
+  '((stream-count u8)
+    (coupled-count u8)
+    (channel-mapping vec (eval emms-info-native--opus-channel-count)))
+  "Opus channel mapping table specification.")
 
 (defconst emms-info-native--opus-comment-header-bindat-spec
   '((opus-tags vec 8)
-    (eval (when (not (equal last emms-info-native--opus-tags-magic-array))
+    (eval (unless (equal last emms-info-native--opus-tags-magic-array)
             (error "Opus framing mismatch: expected ‘%s’, got ‘%s’"
                    emms-info-native--opus-tags-magic-array
                    last)))
@@ -329,43 +413,52 @@ assumed to be valid.")
     (eval (when (> last emms-info-native--max-vorbis-vendor-length)
             (error "Opus vendor length %s is too long" last)))
     (vendor-string vec (vendor-length))
-    (user-comment-list-length u32r)
+    (user-comments-list-length u32r)
     (eval (when (> last emms-info-native--max-num-vorbis-comments)
-            (error "Opus user comment list length %s is too long" last)))
-    (user-comment repeat
-                  (user-comment-list-length)
-                  (struct emms-info-native--vorbis-comment-field-bindat-spec)))
-  "Opus comment header specification.
-Framing is verified.  Too long vendor string and comment list
-will also trigger an error.")
-
-(defconst emms-info-native--opus-headers-bindat-spec
-  '((identification-header struct emms-info-native--opus-identification-header-bindat-spec)
-    (comment-header struct emms-info-native--opus-comment-header-bindat-spec))
-  "Specification for two first Opus header packets.
-They are always an identification header followed by a comment
-header.")
+            (error "Opus user comment list length %s is too long"
+                   last)))
+    (user-comments repeat
+                   (user-comments-list-length)
+                   (struct emms-info-native--vorbis-comment-field-bindat-spec)))
+  "Opus comment header specification.")
 
 ;;;; FLAC code
 
 (defconst emms-info-native--flac-metadata-block-header-bindat-spec
-  '((block-type u8)
-    (block-length u24))
+  '((flags u8)
+    (length u24)
+    (eval (when (or (> last emms-info-native--max-peek-size)
+                    (= last 0))
+            (error "FLAC block length %s is invalid" last))))
   "FLAC metadata block header specification.")
 
-(defconst emms-info-native--flac-comment-bindat-spec
+(defconst emms-info-native--flac-comment-block-bindat-spec
   '((vendor-length u32r)
     (eval (when (> last emms-info-native--max-vorbis-vendor-length)
             (error "FLAC vendor length %s is too long" last)))
     (vendor-string vec (vendor-length))
-    (user-comment-list-length u32r)
+    (user-comments-list-length u32r)
     (eval (when (> last emms-info-native--max-num-vorbis-comments)
-            (error "FLAC user comment list length %s is too long" last)))
-    (user-comment repeat
-                  (user-comment-list-length)
-                  (struct emms-info-native--vorbis-comment-field-bindat-spec)))
-  "FLAC Vorbis comment block specification.
-Too long vendor string and comment list will trigger an error.")
+            (error "FLAC user comment list length %s is too long"
+                   last)))
+    (user-comments repeat
+                   (user-comments-list-length)
+                   (struct emms-info-native--vorbis-comment-field-bindat-spec)))
+  "FLAC Vorbis comment block specification.")
+
+(defun emms-info-native--decode-flac-comments (filename)
+  "Read and decode comments from FLAC file FILENAME.
+Return comments in a list of (FIELD . VALUE) cons cells.  Only
+FIELDs that are listed in
+‘emms-info-native--accepted-vorbis-fields’ are returned."
+  (unless (emms-info-native--has-flac-signature filename)
+    (error "Invalid FLAC stream"))
+  (let* ((block (emms-info-native--decode-flac-comment-block
+                 filename))
+         (unpacked (bindat-unpack emms-info-native--flac-comment-block-bindat-spec
+                                  block))
+         (user-comments (bindat-get-field unpacked 'user-comments)))
+    (emms-info-native--extract-vorbis-comments user-comments)))
 
 (defun emms-info-native--has-flac-signature (filename)
   "Check for FLAC stream marker at the beginning of FILENAME.
@@ -375,117 +468,296 @@ Return t if there is a valid stream marker, nil otherwise."
     (insert-file-contents-literally filename nil 0 4)
     (looking-at "fLaC")))
 
-(defun emms-info-native--flac-decode-block-header (filename offset)
-  "Read and decode FLAC metadata block header from FILENAME starting at OFFSET.
-Return a list (TYPE NEXT-OFFSET LAST).  Here, TYPE is the FLAC
-metadata block type; NEXT-OFFSET is the starting offset of the
-next block; and LAST is t if this was the last metadata block in
-the stream, otherwise nil."
-  (let (block-header
-        block-type
-        block-length
-        end
-        last-flag)
-    (insert-file-contents-literally filename nil offset (+ offset 4) t)
-    (setq offset (+ offset 4))
-    (setq block-header
-          (bindat-unpack emms-info-native--flac-metadata-block-header-bindat-spec
-                         (buffer-string)))
-    (setq block-type
-          (logand (bindat-get-field block-header 'block-type)
-                  #x7F))
-    (setq block-length (bindat-get-field block-header 'block-length))
-    (when (> block-type 6)
-      (error "FLAC block type error: expected <= 6, got %s" block-type))
-    (when (= block-length 0)
-      (error "FLAC block length error: expected >0, got zero"))
-    (setq last-flag (= (logand (bindat-get-field block-header 'block-type)
-                               #x80)
-                       1))
-    (setq end (+ offset block-length))
-    (list block-type end last-flag)))
-
-(defun emms-info-native--flac-decode-comment-block (filename)
-  "Find and decode a comment block from FLAC file FILENAME.
-Return the comment block in a vector.  Trigger an error if any
-metadata block larger than ‘emms-info-native--max-peek-size’ is
-encountered."
+(defun emms-info-native--decode-flac-comment-block (filename)
+  "Read and decode a comment block from FLAC file FILENAME.
+Return the comment block data in a vector."
   (with-temp-buffer
     (set-buffer-multibyte nil)
-    (unless (emms-info-native--has-flac-signature filename)
-      (error "Invalid FLAC stream"))
-    (let ((offset 4)
-          (comment-block (vector))
-          block-type
-          end
-          last-flag)
-      (while (not last-flag)
-        (cl-multiple-value-setq (block-type
-                                 end
-                                 last-flag)
-          (emms-info-native--flac-decode-block-header filename offset))
-        (when (> (- end offset) emms-info-native--max-peek-size)
-          (error "FLAC metadata block is too large: %s" (- end offset)))
-        (when (= block-type 4)
-          ;; Comment block found, extract it.
-          (insert-file-contents-literally filename nil (+ offset 4) end t)
-          (setq comment-block (vconcat (buffer-string))
-                last-flag t))
-        (setq offset end))
+    (let (comment-block
+          last-flag
+          (offset 4))
+      (while (and (not comment-block) (not last-flag))
+        (insert-file-contents-literally filename
+                                        nil
+                                        offset
+                                        (cl-incf offset 4))
+        (let* ((header (bindat-unpack emms-info-native--flac-metadata-block-header-bindat-spec
+                                      (buffer-string)))
+               (end (+ offset (bindat-get-field header 'length)))
+               (flags (bindat-get-field header 'flags))
+               (block-type (logand flags #x7F)))
+          (setq last-flag (> (logand flags #x80) 0))
+          (when (> block-type 6)
+            (error "FLAC block type error: expected ≤ 6, got %s"
+                   block-type))
+          (when (= block-type 4)
+            ;; Comment block found, extract it.
+            (insert-file-contents-literally filename nil offset end t)
+            (setq comment-block (vconcat (buffer-string))))
+          (setq offset end)))
       comment-block)))
 
-(defun emms-info-native--flac-decode-comments (filename)
-  "Read and decode comments from FLAC file FILENAME.
-Return a list of comments.  See
-‘emms-info-native--vorbis-comment-field-bindat-spec’ for comment
-structure."
-  (bindat-get-field (bindat-unpack emms-info-native--flac-comment-bindat-spec
-                                   (emms-info-native--flac-decode-comment-block filename))
-                    'user-comment))
+;;;; id3v2 (MP3) code
+
+(defvar emms-info-native--id3v2-version 0
+  "Last decoded id3v2 version.
+This is a kludge; it is needed because bindat spec cannot refer
+outside itself.")
+
+(defconst emms-info-native--id3v2-magic-array
+  [#x49 #x44 #x33]
+  "id3v2 header magic pattern ‘ID3’.")
+
+(defconst emms-info-native--id3v2-header-bindat-spec
+  '((file-identifier vec 3)
+    (eval (unless (equal last emms-info-native--id3v2-magic-array)
+            (error "id3v2 framing mismatch: expected ‘%s’, got ‘%s’"
+                   emms-info-native--id3v2-magic-array
+                   last)))
+    (version u8)
+    (eval (setq emms-info-native--id3v2-version last))
+    (revision u8)
+    (flags bits 1)
+    (size-bytes vec 4)
+    (size eval (emms-info-native--checked-id3v2-size 'tag last)))
+  "id3v2 header specification.")
+
+(defconst emms-info-native--id3v2-frame-bindat-spec
+  '((id str (eval (if (= emms-info-native--id3v2-version 2) 3 4)))
+    (size-bytes vec (eval (if (= emms-info-native--id3v2-version 2) 3 4)))
+    (size eval (emms-info-native--checked-id3v2-size 'frame last))
+    (flags bits (eval (if (= emms-info-native--id3v2-version 2) 0 2)))
+    (payload vec (size)))
+  "id3v2 frame specification.")
+
+(defconst emms-info-native--id3v2-frame-to-info
+  '(("TP1"  . "artist")
+    ("TPE1" . "artist")
+    ("TPE2" . "albumartist")
+    ("TSOP" . "artistsort")
+    ("TCM"  . "composer")
+    ("TCOM" . "composer")
+    ("TIT2" . "title")
+    ("TT2"  . "title")
+    ("TSOT" . "titlesort")
+    ("TALB" . "album")
+    ("TAL"  . "album")
+    ("TSOA" . "albumsort")
+    ("TRCK" . "tracknumber")
+    ("TRK"  . "tracknumber")
+    ("TPOS" . "discnumber")
+    ("TPA"  . "discnumber")
+    ("TDRC" . "year")
+    ("TDRL" . "year")
+    ("TYER" . "year")
+    ("TYE"  . "year")
+    ("TDOR" . "originalyear")
+    ("TORY" . "originalyear")
+    ("TOR"  . "originalyear"))
+  "Mapping from id3v2 frame identifiers to info fields.")
+
+(defconst emms-info-native--id3v2-text-encodings
+  '((0 . latin-1)
+    (1 . utf-16)
+    (2 . uft-16be)
+    (3 . utf-8))
+  "id3v2 text encodings.")
+
+(defun emms-info-native--decode-id3v2 (filename)
+  "Read and decode id3v2 metadata from FILENAME.
+Return metadata in a list of (FIELD . VALUE) cons cells.  See
+‘emms-info-native--decode-id3v2-text-frame’ for details."
+  (let* (emms-info-native--id3v2-version
+         (header (emms-info-native--decode-id3v2-header filename))
+         (tag-size (bindat-get-field header 'size))
+         (unsync (memq 7 (bindat-get-field header 'flags)))
+         (offset 10))
+    (when (memq 6 (bindat-get-field header 'flags))
+      ;; Skip the extended header.
+      (cl-incf offset
+               (emms-info-native--decode-id3v2-ext-header-size filename)))
+    (emms-info-native--decode-id3v2-frames filename
+                                           offset
+                                           (+ tag-size 10)
+                                           unsync)))
+
+(defun emms-info-native--decode-id3v2-header (filename)
+  "Read and decode id3v2 header from FILENAME."
+  (with-temp-buffer
+    (set-buffer-multibyte nil)
+    (insert-file-contents-literally filename nil 0 10)
+    (bindat-unpack emms-info-native--id3v2-header-bindat-spec
+                   (buffer-string))))
+
+(defun emms-info-native--decode-id3v2-ext-header-size (filename)
+  "Read and decode id3v2 extended header size from FILENAME.
+Return the size.  Signal an error if the size exceeds
+‘emms-info-native--max-peek-size’."
+  (with-temp-buffer
+    (set-buffer-multibyte nil)
+    (insert-file-contents-literally filename nil 10 14)
+    (emms-info-native--checked-id3v2-size (buffer-string))))
+
+(defun emms-info-native--checked-id3v2-size (elt bytes)
+  "Calculate id3v2 element ELT size from BYTES.
+ELT must be either 'tag or 'frame.  Check the validity of size.
+Return the size."
+  (let (size)
+    (cond ((eq elt 'tag)
+           (setq size (emms-info-native--decode-id3v2-size bytes t)))
+          ((eq elt 'frame)
+           (if (= emms-info-native--id3v2-version 4)
+               (setq size (emms-info-native--decode-id3v2-size bytes t))
+             (setq size (emms-info-native--decode-id3v2-size bytes nil)))))
+    (when (or (= size 0) (> size emms-info-native--max-peek-size))
+      (error "id3v2 tag or frame size %s is invalid" size))
+    size))
+
+(defun emms-info-native--decode-id3v2-size (bytes syncsafe)
+  "Decode id3v2 element size from BYTES.
+Depending on SYNCSAFE, BYTES are interpreted as 7- or 8-bit
+bytes, MSB first.
+
+Return the decoded size."
+  (let ((num-bits (if syncsafe 7 8)))
+    (apply '+ (seq-map-indexed (lambda (elt idx)
+                                 (* (expt 2 (* num-bits idx)) elt))
+                               (reverse bytes)))))
+
+(defun emms-info-native--decode-id3v2-frames (filename
+                                              begin
+                                              end
+                                              unsync)
+  "Read and decode id3v2 text frames from FILENAME.
+BEGIN should be the offset of first byte after id3v2 header and
+extended header (if any), and END should be the offset after the
+complete id3v2 tag.
+
+If UNSYNC is t, the frames are assumed to have gone through
+unsynchronization and decoded as such.
+
+Return metadata in a list of (FIELD . VALUE) cons cells.  See
+‘emms-info-native--decode-id3v2-text-frame’ for details."
+  (with-temp-buffer
+    (set-buffer-multibyte nil)
+    (insert (emms-info-native--read-id3v2-frames filename
+                                                 begin
+                                                 end
+                                                 unsync))
+    (emms-info-native--decode-id3v2-text-frames (buffer-string))))
+
+(defun emms-info-native--read-id3v2-frames (filename begin end unsync)
+  "Read id3v2 frames from FILE.
+Start at offset BEGIN and end before offset END.  If UNSYNC is t,
+reverse unsynchronization.
+
+Return the frames."
+  (with-temp-buffer
+    (set-buffer-multibyte nil)
+    (insert-file-contents-literally filename nil begin end)
+    (when unsync
+      (while (re-search-forward (string 255 0) nil t)
+        (replace-match (string 255))))
+    (buffer-string)))
+
+(defun emms-info-native--decode-id3v2-text-frames (frames)
+  "Decode id3v2 text frames from FRAMES.
+
+Return metadata in a list of (FIELD . VALUE) cons cells.  See
+‘emms-info-native--decode-id3v2-text-frame’ for details."
+  (let (comments
+        (offset 0))
+    (condition-case nil
+        (while (< offset (length frames))
+          (let* ((frame (bindat-unpack emms-info-native--id3v2-frame-bindat-spec
+                                       frames
+                                       offset))
+                 (comment (emms-info-native--decode-id3v2-text-frame
+                           frame)))
+            (when comment (push comment comments))
+            (cl-incf offset
+                     (+ (bindat-get-field frame 'size)
+                        (if (= emms-info-native--id3v2-version 2) 6 10)))))
+      (error nil))
+    comments))
+
+(defun emms-info-native--decode-id3v2-text-frame (frame)
+  "Identify and decode id3v2 text frame FRAME.
+If FRAME’s identifier matches a key in
+‘emms-info-native--id3v2-frame-to-info’, return a cons cell
+(FIELD . VALUE), where FIELD is the corresponding info field
+identifier and VALUE is the decoded text.  Otherwise return nil."
+  (let ((info-id (emms-info-native--id3v2-frame-info-id frame))
+        (payload (bindat-get-field frame 'payload)))
+    (when info-id
+      (cons info-id (emms-info-native--decode-id3v2-string payload)))))
+
+(defun emms-info-native--id3v2-frame-info-id (frame)
+  "Return the emms-info identifier for FRAME.
+If there is no such identifier, return nil."
+  (cdr (assoc (bindat-get-field frame 'id)
+              emms-info-native--id3v2-frame-to-info)))
+
+(defun emms-info-native--decode-id3v2-string (bytes)
+  "Decode id3v2 text information from BYTES.
+Remove the terminating null byte, if any.  Return the text as
+string."
+  (let* ((encoding (emms-info-native--id3v2-text-encoding bytes))
+         (string (mapconcat #'byte-to-string (seq-rest bytes) ""))
+         (decoded (decode-coding-string string encoding)))
+    (when (> (length decoded) 0)
+      (if (equal (substring decoded -1) "\0")
+          (substring decoded 0 -1)
+        decoded))))
+
+(defun emms-info-native--id3v2-text-encoding (bytes)
+  "Return the encoding for text information BYTES."
+  (cdr (assoc (seq-first bytes)
+              emms-info-native--id3v2-text-encodings)))
 
 ;;;; EMMS code
 
+(defun emms-info-native (track)
+  "Set info fields for TRACK.
+Supports Ogg Vorbis/Opus, FLAC, and MP3 files."
+  (let* ((filename (emms-track-name track))
+         (info-fields (emms-info-native--decode-info-fields filename)))
+    (dolist (field info-fields)
+      (let ((name (intern (concat "info-" (car field))))
+            (value (cdr field)))
+        (emms-track-set track
+                        name
+                        (if (eq name 'info-playing-time)
+                            (string-to-number value)
+                          value))))))
+
+(defun emms-info-native--decode-info-fields (filename)
+  "Decode info fields from FILENAME.
+Return a list of (FIELD . VALUE) cons cells, where FIELD is an
+info field and VALUE is the corresponding info value.  Both are
+strings."
+  (let ((stream-type (emms-info-native--find-stream-type filename)))
+    (cond ((or (eq stream-type 'vorbis) (eq stream-type 'opus))
+           (emms-info-native--decode-ogg-comments filename stream-type))
+          ((eq stream-type 'flac)
+           (emms-info-native--decode-flac-comments filename))
+          ((eq stream-type 'mp3)
+           (emms-info-native--decode-id3v2 filename))
+          (t nil))))
+
 (defun emms-info-native--find-stream-type (filename)
   "Deduce the stream type from FILENAME.
 This is a naive implementation that relies solely on filename
 extension.
 
-Return one of symbols ‘vorbis’, ‘opus’, or ‘flac’."
+Return one of symbols ‘vorbis’, ‘opus’, ‘flac’, or ‘mp3’."
   (let ((case-fold-search t))
     (cond ((string-match ".ogg$" filename) 'vorbis)
           ((string-match ".opus$" filename) 'opus)
           ((string-match ".flac$" filename) 'flac)
+          ((string-match ".mp3$" filename) 'mp3)
           (t nil))))
 
-(defun emms-info-native (track)
-  "Set info fields for TRACK.
-Supports Ogg Vorbis/Opus and FLAC files.
-
-Return t if TRACK was updated, nil otherwise."
-  (let* ((filename (emms-track-name track))
-         (stream-type (emms-info-native--find-stream-type filename))
-         (comments)
-         update-flag)
-    (setq comments
-          (cond ((or (eq stream-type 'vorbis) (eq stream-type 'opus))
-                 (emms-info-native--ogg-decode-comments filename stream-type))
-                ((eq stream-type 'flac)
-                 (emms-info-native--flac-decode-comments filename))
-                (t nil)))
-    (dolist (comment comments)
-      (let ((pair (emms-info-native--split-vorbis-comment
-                   (cdr (assoc 'user-comment comment)))))
-        (when pair
-          (let ((name (intern-soft (concat "info-" (downcase (car pair)))))
-                (value (cdr pair)))
-            (setq update-flag (or update-flag name))
-            (emms-track-set track
-                            name
-                            (if (eq name 'info-playing-time)
-                                (string-to-number value)
-                              value))))))
-    update-flag))
-
 (provide 'emms-info-native)
 
 ;;; emms-info-native.el ends here