Summerize/internal/clip/clip.go

// Package clip selects the best 60–90s window from a timestamped transcript
// (using a Summarizer to do the picking) and runs ffmpeg to cut that window
// out of the original media.
package clip

import (
	"context"
	"encoding/json"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"

	"publish/internal/summarize"
	"publish/internal/transcribe"
)

// Selection is the LLM's chosen clip window plus metadata.
type Selection struct {
	StartSeconds float64 `json:"start_seconds"`
	EndSeconds   float64 `json:"end_seconds"`
	Title        string  `json:"title"`
	Hook         string  `json:"hook"`
	Quote        string  `json:"quote"`
	Reasoning    string  `json:"reasoning"`
}

// Duration returns the selected window length in seconds.
func (s Selection) Duration() float64 { return s.EndSeconds - s.StartSeconds }

// Pick asks the summarizer to choose the best window in the given segments,
// using promptTemplate (which may contain {{MIN_SECONDS}} / {{MAX_SECONDS}}
// placeholders). It clamps and validates the returned window against minSec
// and maxSec.
func Pick(ctx context.Context, sum summarize.Summarizer, promptTemplate string, segs []transcribe.Segment, minSec, maxSec float64) (Selection, string, error) {
	if len(segs) == 0 {
		return Selection{}, "", fmt.Errorf("no transcript segments to choose from")
	}
	prompt := strings.NewReplacer(
		"{{MIN_SECONDS}}", fmt.Sprintf("%g", minSec),
		"{{MAX_SECONDS}}", fmt.Sprintf("%g", maxSec),
	).Replace(promptTemplate)

	body := transcribe.FormatForLLM(segs)

	raw, err := sum.Summarize(ctx, prompt, body)
	if err != nil {
		return Selection{}, "", err
	}

	jsonText, err := extractJSONObject(raw)
	if err != nil {
		return Selection{}, raw, fmt.Errorf("could not find JSON object in model output: %w", err)
	}
	var sel Selection
	if err := json.Unmarshal([]byte(jsonText), &sel); err != nil {
		return Selection{}, raw, fmt.Errorf("parsing selection JSON: %w\n--- raw ---\n%s", err, jsonText)
	}

	if err := validate(&sel, segs, minSec, maxSec); err != nil {
		return sel, raw, err
	}
	return sel, raw, nil
}

func validate(sel *Selection, segs []transcribe.Segment, minSec, maxSec float64) error {
	if sel.EndSeconds <= sel.StartSeconds {
		return fmt.Errorf("invalid window: end (%g) <= start (%g)", sel.EndSeconds, sel.StartSeconds)
	}
	maxEnd := segs[len(segs)-1].End
	if sel.StartSeconds < 0 || sel.EndSeconds > maxEnd+1.0 {
		return fmt.Errorf("window [%g, %g] is outside transcript bounds [0, %g]",
			sel.StartSeconds, sel.EndSeconds, maxEnd)
	}
	dur := sel.Duration()
	// Allow small slop on either side; otherwise reject.
	if dur < minSec-2 || dur > maxSec+2 {
		return fmt.Errorf("window duration %.1fs is outside requested bounds [%g, %g]",
			dur, minSec, maxSec)
	}
	return nil
}

// extractJSONObject pulls the first balanced {...} object out of s, ignoring
// braces that appear inside JSON strings. Useful when the model wraps its
// answer in prose despite being told not to.
func extractJSONObject(s string) (string, error) {
	start := strings.Index(s, "{")
	if start < 0 {
		return "", fmt.Errorf("no '{' in response")
	}
	depth := 0
	inStr := false
	esc := false
	for i := start; i < len(s); i++ {
		c := s[i]
		if inStr {
			switch {
			case esc:
				esc = false
			case c == '\\':
				esc = true
			case c == '"':
				inStr = false
			}
			continue
		}
		switch c {
		case '"':
			inStr = true
		case '{':
			depth++
		case '}':
			depth--
			if depth == 0 {
				return s[start : i+1], nil
			}
		}
	}
	return "", fmt.Errorf("unbalanced braces")
}

// portraitFilter center-crops any source aspect ratio to a 9:16 sub-rectangle
// (no distortion, just cropping) and scales to 1080x1920. The min() expressions
// pick the largest 9:16 box that fits inside the source: 16:9 sources lose the
// left/right edges, 9:16 sources are unchanged, and 4:3 / 1:1 sources crop the
// sides. setsar=1 forces square pixels.
const portraitFilter = `crop=min(iw\,ih*9/16):min(ih\,iw*16/9),scale=1080:1920,setsar=1`

// MaxClipBytes is the hard size ceiling enforced by ffmpeg's -fs flag.
// Realistic 60–90s 1080x1920 H.264 clips at CRF 23 land 30–100 MB, so this is
// a safety cap rather than a target.
const MaxClipBytes = 1 << 30 // 1 GiB

// Extract runs ffmpeg to cut [start, end) seconds out of input into outPath.
// For video inputs, the clip is re-encoded as a 1080x1920 portrait (9:16
// center-crop) under a 1 GiB size cap. If reencode is false, stream copy is
// used (fast, keyframe-aligned, but the source aspect ratio is preserved).
func Extract(ctx context.Context, input string, sel Selection, outPath string, reencode bool) error {
	if _, err := exec.LookPath("ffmpeg"); err != nil {
		return fmt.Errorf("ffmpeg not on PATH: %w", err)
	}
	if err := os.MkdirAll(filepath.Dir(outPath), 0o755); err != nil {
		return err
	}

	dur := sel.EndSeconds - sel.StartSeconds
	args := []string{
		"-y",
		"-loglevel", "error",
		"-ss", fmt.Sprintf("%.3f", sel.StartSeconds),
		"-i", input,
		"-t", fmt.Sprintf("%.3f", dur),
	}
	if reencode {
		if hasVideoExt(input) {
			args = append(args,
				"-vf", portraitFilter,
				"-c:v", "libx264",
				"-preset", "fast",
				"-crf", "23",
				"-c:a", "aac",
				"-b:a", "128k",
				"-movflags", "+faststart",
			)
		} else {
			args = append(args,
				"-vn",
				"-c:a", "aac",
				"-b:a", "128k",
			)
		}
	} else {
		args = append(args, "-c", "copy")
	}
	args = append(args, "-fs", fmt.Sprintf("%d", MaxClipBytes), outPath)

	cmd := exec.CommandContext(ctx, "ffmpeg", args...)
	cmd.Stdout = os.Stderr
	cmd.Stderr = os.Stderr
	if err := cmd.Run(); err != nil {
		return fmt.Errorf("ffmpeg cut: %w", err)
	}
	return nil
}

func hasVideoExt(p string) bool {
	switch strings.ToLower(filepath.Ext(p)) {
	case ".mp4", ".mov", ".mkv", ".webm", ".avi", ".m4v", ".flv", ".ts":
		return true
	}
	return false
}

// DefaultOutputPath builds <input-without-ext>.clip<ext> for video inputs and
// .m4a for audio inputs.
func DefaultOutputPath(input string) string {
	base := strings.TrimSuffix(input, filepath.Ext(input))
	if hasVideoExt(input) {
		return base + ".clip" + filepath.Ext(input)
	}
	return base + ".clip.m4a"
}