js: fix f♯maj scale display by using e♯ on the 7th.

1 files changed, 19 insertions, 4 deletions

diff --git a/scale.mjs b/scale.mjs
index b4f043d..5341413 100644
--- a/scale.mjs
+++ b/scale.mjs
@@ -8,6 +8,16 @@ export const chromaticScale = new Proxy(
     ['A', 'A♯', 'B', 'C', 'C♯', 'D', 'D♯', 'E', 'F', 'F♯', 'G', 'G♯'],
     ringHandler);
 
+function notesBetween(a, b) {
+    const [tonicA, tonicB] = [a[0], b[0]]
+    const abs = tonicB.charCodeAt() - tonicA.charCodeAt();
+    if (abs < 0) {
+        return abs + 7;
+    } else {
+        return abs;
+    }
+}
+
 function scaleFromIntervals(tonic, intervals) {
     const scaleIndex = chromaticScale.indexOf(tonic);
     if (scaleIndex < 0) {
@@ -23,14 +33,19 @@ function scaleFromIntervals(tonic, intervals) {
         const note = chromaticScale[scaleIndex + steps];
         // don't display two base notes in a row by changing
         // accidentals.
-        if (note[0] === lastBase) {
+        const delta = notesBetween(lastBase, note);
+        if (delta === 0) {
             const nextBase = chromaticScale[scaleIndex + steps + 1][0];
             scale.push(`${nextBase}♭`)
-            lastBase = nextBase[0];
+        } else if (delta === 1) {
+            scale.push(`${note}`)
         } else {
-            scale.push(note);
-            lastBase = note[0];
+            // we can only be two away, so take the sharp of the
+            // previous note.
+            const nextBase = chromaticScale[scaleIndex + steps - 1][0];
+            scale.push(`${nextBase}♯`)
         }
+        lastBase = scale[scale.length-1];
     }
     return new Proxy(scale, ringHandler);
 }