From c5fa413323dc10481715b5fcb24355934557fcff Mon Sep 17 00:00:00 2001
From: hermes <hermes@obahan.xyz>
Date: Wed, 30 Sep 2026 21:59:31 +0200
Subject: [PATCH] add internal/domain/playlist.go

---
 internal/domain/playlist.go | 118 ++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 internal/domain/playlist.go

diff --git a/internal/domain/playlist.go b/internal/domain/playlist.go
new file mode 100644
index 0000000..e074db6
--- /dev/null
+++ b/internal/domain/playlist.go
@@ -0,0 +1,118 @@
+// Package domain holds the core radio model: tracks and playlist policy.
+// No I/O, no dependencies — pure rules, fully unit-tested.
+package domain
+
+import (
+	"math"
+	"math/rand"
+	"sort"
+	"strconv"
+	"time"
+)
+
+// UnixTime is a time.Time that (un)marshals as a numeric Unix epoch,
+// matching the generator app's Python `time.time()` float format.
+type UnixTime struct{ time.Time }
+
+func (u *UnixTime) UnmarshalJSON(b []byte) error {
+	s := string(b)
+	if s == "null" {
+		u.Time = time.Time{}
+		return nil
+	}
+	// quoted RFC3339 (defensive)
+	if len(s) > 1 && s[0] == '"' {
+		t, err := time.Parse(time.RFC3339, s[1:len(s)-1])
+		if err == nil {
+			u.Time = t
+			return nil
+		}
+	}
+	f, err := strconv.ParseFloat(s, 64)
+	if err != nil {
+		return err
+	}
+	sec := math.Floor(f)
+	nsec := int64((f - sec) * 1e9)
+	u.Time = time.Unix(int64(sec), nsec)
+	return nil
+}
+
+func (u UnixTime) MarshalJSON() ([]byte, error) {
+	return []byte(strconv.FormatFloat(float64(u.UnixNano())/1e9, 'f', 3, 64)), nil
+}
+
+// Track is a generated song.
+type Track struct {
+	ID        string   `json:"id"`
+	Genre     string   `json:"genre"`
+	Language  string   `json:"language"`
+	Title     string   `json:"title"`
+	Style     string   `json:"style"`
+	Filename  string   `json:"filename"` // relative to library dir, e.g. "synthwave/<id>.flac"
+	Duration  float64  `json:"duration"`
+	CreatedAt UnixTime `json:"created_at"`
+	Source    string   `json:"source"` // generated | library
+	Rating    int      `json:"rating"` // 1 liked, -1 disliked, 0 unrated
+}
+
+// DisplayTitle is what the stereo display should show.
+func (t Track) DisplayTitle() string {
+	if t.Title != "" {
+		return t.Title
+	}
+	return t.Style
+}
+
+// Playable reports whether the track may be broadcast.
+func (t Track) Playable() bool { return t.Rating != -1 }
+
+// FilterPlayable drops disliked tracks.
+func FilterPlayable(tracks []Track) []Track {
+	out := make([]Track, 0, len(tracks))
+	for _, t := range tracks {
+		if t.Playable() {
+			out = append(out, t)
+		}
+	}
+	return out
+}
+
+// BuildPlaylist returns a newest-biased random sample of at most size tracks.
+// Weight of the k-th newest track is 1/(k+bias): low bias = strongly favors
+// fresh songs; bias >> pool size = near-uniform shuffle.
+// seed makes the selection deterministic for tests.
+func BuildPlaylist(tracks []Track, size, bias int, seed int64) []Track {
+	pool := FilterPlayable(tracks)
+	sort.Slice(pool, func(i, j int) bool { return pool[i].CreatedAt.After(pool[j].CreatedAt.Time) })
+	if size > len(pool) {
+		size = len(pool)
+	}
+	if size <= 0 {
+		return nil
+	}
+	rng := rand.New(rand.NewSource(seed))
+	remaining := make([]Track, len(pool))
+	copy(remaining, pool)
+
+	out := make([]Track, 0, size)
+	for len(out) < size && len(remaining) > 0 {
+		weights := make([]float64, len(remaining))
+		total := 0.0
+		for k := range remaining {
+			weights[k] = 1.0 / float64(k+bias)
+			total += weights[k]
+		}
+		r := rng.Float64() * total
+		acc := 0.0
+		for k := range remaining {
+			acc += weights[k]
+			if r <= acc {
+				out = append(out, remaining[k])
+				remaining = append(remaining[:k], remaining[k+1:]...)
+				break
+			}
+		}
+	}
+	return out
+}