gistfile1.txt

#!/usr/bin/env python3
"""
純粋 Python だけで **SMF (Standard MIDI File)-format 0** を書き出す最小サンプル。
 - MThd / MTrk チャンクを手動組み立て
 - Set-Tempo → Note On → Note Off → End-of-Track という典型的な並び
 - 可変長数量 (VLQ) も自前実装
gist に貼り付けてそのまま動かせます（`example.mid` が生成されます）。
"""

import struct
from typing import List


def write_vlq(value: int) -> bytes:
    """Variable-Length Quantity (最大 4 byte) を big-endian で符号化"""
    bytes_: List[int] = [value & 0x7F]
    value >>= 7
    while value:
        bytes_.insert(0, (value & 0x7F) | 0x80)  # 先頭側に 1 で継続ビットを立てる
        value >>= 7
    return bytes(bytes_)


def create_midi(
    path: str = "example.mid",
    ticks_per_quarter: int = 480,
    tempo_us_per_qn: int = 500_000,  # 120 BPM
    note: int = 60,                  # Middle-C
    velocity: int = 64,
    duration_ticks: int = 480        # 4 分音符
) -> None:
    """単一トラックに 1 音だけ鳴らす format 0 ファイルを生成"""

    events = bytearray()

    # --- Meta: Set Tempo (推奨) ------------------------------
    events += write_vlq(0)                 # Δ-time
    events += b"\xFF\x51\x03"              # type 0x51, length 3
    events += struct.pack(">I", tempo_us_per_qn)[1:]  # 上位 3 byte

    # --- Note On --------------------------------------------
    events += write_vlq(0)                 # Δ-time
    events += b"\x90"                      # 0x9n, ch = 0
    events += bytes([note, velocity])

    # --- Note Off -------------------------------------------
    events += write_vlq(duration_ticks)    # Δ-time
    events += b"\x80"                      # 0x8n, ch = 0
    events += bytes([note, 0])

    # --- End-of-Track (必須) ---------------------------------
    events += write_vlq(0)                 # Δ-time
    events += b"\xFF\x2F\x00"

    # ---- チャンク組み立て ----------------------------------
    track_chunk = b"MTrk" + struct.pack(">I", len(events)) + events
    header_chunk = (
        b"MThd"
        + struct.pack(">IHHH",
                      6,       # 以降のヘッダ長
                      0,       # format = 0
                      1,       # track 数
                      ticks_per_quarter)
    )

    with open(path, "wb") as f:
        f.write(header_chunk)
        f.write(track_chunk)


if __name__ == "__main__":
    create_midi()