size-match-audio.sh

#!/usr/bin/env bash

# ==========================================
# Configuration & Helpers
# ==========================================
VIDEO_IN="$1"
AUDIO_SRC="${2:-${VIDEO_IN%.*}.mp4}"
TARGET_MB="${3:-20}"

# Check dependencies
for cmd in ffmpeg ffprobe bc; do
    if ! command -v $cmd &> /dev/null; then
        echo "Error: '$cmd' is not installed."
        exit 1
    fi
done

if [[ -z "$VIDEO_IN" || ! -f "$VIDEO_IN" || ! -f "$AUDIO_SRC" ]]; then
    echo "Usage: $0 <input_video> [audio_source_mp4] [target_size_mb]"
    exit 1
fi

# Opus bitrates constraints
MIN_BITRATE=12000
MAX_BITRATE=512000

TARGET_SIZE_BYTES=$(echo "$TARGET_MB * 1048576" | bc)
TEMP_FILE=".tmp_remux_$$.webm"
trap 'rm -f "$TEMP_FILE"' EXIT INT TERM

# ==========================================
# 1. Analyze Components
# ==========================================
VIDEO_SIZE_BYTES=$(ffprobe -v error -select_streams v:0 -show_entries packet=size -bitexact -of csv=p=0 "$VIDEO_IN" | awk '{sum+=$1} END {print sum}')
DURATION=$(ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "$VIDEO_IN")

VIDEO_MB=$(echo "scale=2; $VIDEO_SIZE_BYTES / 1048576" | bc)
echo "Video Size: $VIDEO_MB MB and $DURATION seconds."

# Estimate initial bitrate to get into the ballpark
AVAILABLE_BYTES=$(echo "$TARGET_SIZE_BYTES - $VIDEO_SIZE_BYTES" | bc)
INITIAL_BITRATE=$(echo "($AVAILABLE_BYTES * 8) / $DURATION" | bc)

# ==========================================
# 2. Refined Binary Search
# ==========================================
# Define range +/- 20% of the initial estimate
LOW=$(echo "$INITIAL_BITRATE * 0.8" / 1 | bc)
HIGH=$(echo "$INITIAL_BITRATE * 1.2" / 1 | bc)
BEST_BITRATE=$INITIAL_BITRATE
ITERATIONS=16 # Sufficient for bitrate precision

echo "Optimizing audio bitrate with ${ITERATIONS}-iteration refinement..."

[ "$LOW" -lt "$MIN_BITRATE" ] && LOW=$MIN_BITRATE
[ "$HIGH" -gt "$MAX_BITRATE" ] && HIGH=$MAX_BITRATE

# If constraints make search range invalid, just set to max or min
[ "$LOW" -gt "$HIGH" ] && LOW=$MAX_BITRATE && HIGH=$MAX_BITRATE

for ((i=1; i<=ITERATIONS; i++)); do
    MID=$(( (LOW + HIGH) / 2 ))

    # Run ffmpeg and capture the exit code
    ffmpeg -nostdin -y -i "$VIDEO_IN" -i "$AUDIO_SRC" \
        -map 0:v:0 -map 1:a \
        -c:v copy -c:a libopus -b:a "${MID}" \
        "$TEMP_FILE" > /dev/null 2>&1

    _EXIT_CODE=$?

    # Only process results if ffmpeg succeeded
    if [ $_EXIT_CODE -eq 0 ]; then
        FILE_SIZE=$(stat -c%s "$TEMP_FILE")

        if [ "$FILE_SIZE" -le "$TARGET_SIZE_BYTES" ]; then
            DIFF=$(( TARGET_SIZE_BYTES - FILE_SIZE ))
            BEST_BITRATE=$MID
            printf "\r[Iter %2d/%d] Captured best: %dbps... (diff: %d)" "$i" "$ITERATIONS" "$MID" "$DIFF"
            LOW=$(( MID + 1 ))
        else
            HIGH=$(( MID - 1 ))
        fi
    else
        echo "Warning: FFmpeg failed at bitrate ${MID}. Skipping iteration."
        # Optionally adjust your bounds or break if the failure is critical
        HIGH=$(( MID - 1 ))
    fi
done
echo ""

# Final check
FINAL_SIZE=$(stat -c%s "$TEMP_FILE")
if [ "$FINAL_SIZE" -gt "$TARGET_SIZE_BYTES" ]; then
    echo "Warning: Even at the lowest bitrate, the file exceeds the target size."
fi

# ==========================================
# 3. Final Output
# ==========================================
FILENAME=$(basename "${VIDEO_IN%.*}")
OUTPUT="audio_optimized_${FILENAME}_${TARGET_MB}MB.webm"

ffmpeg -nostdin -y -i "$VIDEO_IN" -i "$AUDIO_SRC" \
    -map 0:v:0 -map 1:a \
    -c:v copy -c:a libopus -b:a "${BEST_BITRATE}" \
    "$OUTPUT" > /dev/null 2>&1

BEST_BITRATE_KB=$(( BEST_BITRATE / 1024 ))
FINAL_MB=$(echo "scale=2; $(stat -c%s "$OUTPUT") / 1048576" | bc)

echo "------------------------------------------------"
echo "Success! Final Size: ${FINAL_MB} MB (target: ${TARGET_MB} MB)"
echo "Selected Audio Bitrate: ${BEST_BITRATE_KB} kbps (${BEST_BITRATE})"
echo "Saved to: $OUTPUT"
echo "------------------------------------------------"