refactor: backend-only transpose, remove client-side transpose logic

app/app/lib/api.ts

@@ -94,9 +94,16 @@ export async function listSongs(q = "", sort = "date", order = "desc"): Promise<
   return res.json();
 }
 
-export async function getSong(id: string, applyCapo = false): Promise<Song | null> {
-  const url = applyCapo
-    ? `${API_BASE}/songs/${id}?apply_capo=true`
+export async function getSong(
+  id: string,
+  opts: { applyCapo?: boolean; transpose?: number } = {},
+): Promise<Song | null> {
+  const params = new URLSearchParams();
+  if (opts.applyCapo) params.set("apply_capo", "true");
+  if (opts.transpose && opts.transpose !== 0)
+    params.set("transpose", String(opts.transpose));
+  const url = params.size
+    ? `${API_BASE}/songs/${id}?${params}`
     : `${API_BASE}/songs/${id}`;
   const res = await fetch(url);
   if (res.status === 404) return null;

app/app/lib/transpose.ts

@@ -1,34 +0,0 @@
-import type { Song } from "./types";
-
-const NOTES_SHARP = ["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"];
-const NOTES_FLAT  = ["C", "Db", "D", "Eb", "E", "F", "Gb", "G", "Ab", "A", "Bb", "B"];
-
-function transposeChord(chord: string, semitones: number): string {
-  const match = chord.match(/^([A-G][#b]?)(.*)/);
-  if (!match) return chord;
-  const [, root, descriptor] = match;
-  const idx = NOTES_SHARP.indexOf(root) !== -1
-    ? NOTES_SHARP.indexOf(root)
-    : NOTES_FLAT.indexOf(root);
-  if (idx === -1) return chord;
-  const newIdx = ((idx + semitones) % 12 + 12) % 12;
-  const notes = semitones >= 0 ? NOTES_SHARP : NOTES_FLAT;
-  return notes[newIdx] + descriptor;
-}
-
-export function transposeSong(song: Song, semitones: number): Song {
-  if (semitones === 0) return song;
-  return {
-    ...song,
-    sections: song.sections.map((section) => ({
-      ...section,
-      lines: section.lines.map((line) => ({
-        ...line,
-        chords: line.chords.map((cp) => ({
-          ...cp,
-          chord: transposeChord(cp.chord, semitones),
-        })),
-      })),
-    })),
-  };
-}

app/app/routes/songs.$id.tsx

@@ -11,7 +11,6 @@ import { EditSongSheet } from "~/components/edit-song-sheet";
 import { DeleteSongDialog } from "~/components/delete-song-dialog";
 import { SectionNav } from "~/components/section-nav";
 import { AutoScrollControls } from "~/components/auto-scroll-controls";
-import { transposeSong } from "~/lib/transpose";
 import { extractUniqueChords } from "~/lib/song-utils";
 import { getSong } from "~/lib/api";
 import { useAuth } from "~/lib/auth";
@@ -82,23 +81,19 @@ export default function SongDetail() {
 
   useEffect(() => {
     setLoading(true);
-    getSong(id)
+    getSong(id, { transpose: initOffset })
       .then((s) => {
         setBaseSong(s);
         setDisplayedSong(s);
       })
       .finally(() => setLoading(false));
-  }, [id]);
+  }, [id]); // eslint-disable-line
 
   useEffect(() => {
-    if (applyCapo && baseSong?.meta.capo) {
-      getSong(id, true).then((s) => {
-        if (s) setDisplayedSong(s);
-      });
-    } else {
-      setDisplayedSong(baseSong);
-    }
-  }, [applyCapo]); // eslint-disable-line
+    getSong(id, { applyCapo, transpose: offset }).then((s) => {
+      if (s) setDisplayedSong(s);
+    });
+  }, [id, applyCapo, offset]);
 
   function handleOffsetChange(newOffset: number) {
     setOffset(newOffset);
@@ -158,9 +153,8 @@ export default function SongDetail() {
     );
   }
 
-  const displayed = transposeSong(displayedSong, offset);
-  const uniqueChords = extractUniqueChords(displayed.sections);
-  const sectionItems = displayed.sections.map((s, i) => ({
+  const uniqueChords = extractUniqueChords(displayedSong.sections);
+  const sectionItems = displayedSong.sections.map((s, i) => ({
     label: s.label,
     index: i,
   }));
@@ -200,7 +194,7 @@ export default function SongDetail() {
         >
           <div className="max-w-lg mx-auto lg:max-w-none">
             <ChordChart
-              sections={displayed.sections}
+              sections={displayedSong.sections}
               fontSize={fontSize}
               onChordClick={handleChordClick}
             />
@@ -224,7 +218,6 @@ export default function SongDetail() {
         </div>
       </div>
 
-      {/* Bottom toolbar: section nav + auto-scroll */}
       <div className="lg:hidden border-t border-border bg-background flex items-center">
         <SectionNav sections={sectionItems} onJump={handleSectionJump} />
         <AutoScrollControls scrollRef={scrollRef} />

crates/api-types/src/lib.rs

@@ -29,6 +29,7 @@ pub struct UpdateSongRequest {
 #[derive(Deserialize, ToSchema, IntoParams)]
 pub struct GetSongQuery {
     pub apply_capo: Option<bool>,
+    pub transpose: Option<i8>,
 }
 
 #[derive(Deserialize, ToSchema)]

crates/presentation/src/routes/songs.rs

@@ -155,6 +155,13 @@ pub async fn get_song(
         song
     };
 
+    let song = match params.transpose {
+        Some(semitones) if semitones != 0 => {
+            ChordTransposer.transpose_song(&song, semitones)
+        }
+        _ => song,
+    };
+
     Ok(Json(song))
 }