sdlamp2/docs/sdlamp2-fsd.md

SDLamp2 - Functional Specification Document

Document Information

Field	Value
Version	1.0
Status	Draft
Created	2026-02-10
Updated	2026-02-10

1. Purpose

This document specifies the functional requirements for an SDL2 based media player application written in C, aimed at being used by a pre-teen child. The name is a reference to the Winamp media player, popular in the 90's and inspiration can be drawn from that.

2. Goals

• Play large m4a audio files, approximately 3 hours each, created from original cassette tapes that contain collections of fairy tales for children
• Present a super simple interface, easy to use by a child less than 10 years old, reminiscent of a cassette player: rewind, stop, play / pause, fast forward, load another tape
• The playback of the audio files must emulate that of cassette tapes, meaning the position of each file should be stored and remembered and playback should resume from that position even if other files have been played in the mean time
• The interface should show the embedded album cover if present in the file

3. Software architecture

• The program should be written in modern C using GCC or Clang
• It should use the SDL2 library for screen rendering, audio playback and input handling
• It should use the libav (ffmpeg) suite of libraries to decode m4a files and potentially other formats (e.g. mp3)
• It should not call out to an ffmpeg binary, but instead use the libav C API library functions

4. Design principles

• Version control (git) must be used
• Compilation should be performed by a simple shell script or batch file, not a complicated build system like make or cmake
• C source code files should be formatted using "Google" style with an additional change of ColumnLimit set to 100
• Less is more, minimize dependencies, avoid pulling in extra libraries, always talk through with owner first
• Keep it simple, apply Casey Muratori's semantic compression principles, don't refactor too soon or write code that's too clever for its own good
• Keep a changelog in this functional specification document

5. Changelog

2026-02-10 — Full implementation of audio player features

• Streaming decoder: Replaced fire-and-forget decode_audio() with a persistent Decoder struct that streams audio on demand via decoder_pump(). Uses libswresample (swr_alloc_set_opts2) to convert from the decoder's native format (e.g. planar float) to interleaved float stereo 48kHz. Fixes the sped-up/distorted audio bug and eliminates the multi-GB memory spike for long files.
• Seeking: Rewind (10s back) and fast-forward (10s ahead) via av_seek_frame() with codec buffer flush and audio pipeline clear. Clamped to file bounds.
• Play/Stop separation: Removed play/pause toggle. Play always resumes, stop always pauses in place and saves position. No icon toggling.
• Position persistence: Saves/loads playback position per file in positions.txt (tab-separated) in the audio directory. Position saved on stop, quit, and file switch. Restored on file open.
• File selection: Scans audio directory for .m4a, .mp3, .wav, .ogg files. Sorted alphabetically. 5th button ("next tape") cycles through files. Window title shows current filename.
• Album art: Extracts embedded cover art (AV_DISPOSITION_ATTACHED_PIC) and displays it scaled with preserved aspect ratio in the upper portion of the window.
• Progress bar: Gray bar between album art and controls showing playback position relative to duration.
• Command-line argument: First argument sets audio directory (defaults to current working directory).
• Error handling: Non-fatal errors (stream ops, corrupt files) use fprintf(stderr) and continue. Corrupt files are skipped when switching. Fatal errors (SDL init, window, audio device) still abort. Proper cleanup order on exit.
• EOF handling: When a file plays to the end, playback auto-pauses and resets to the start.
• Removed dead code: load_audio_file(), wavbuf/wavlen/wavspec globals.