android/app/src/main/java/com/fabledsword/thoughtsync/ui/DerivedTint.kt

@@ -0,0 +1,93 @@
+package com.fabledsword.thoughtsync.ui
+
+// The colour a note has when nothing chose one for it.
+//
+// A board of `default` notes is a wall of white rectangles and the eye gets no help
+// telling one from the next. Every note now carries some tint; this is where an
+// untagged one gets it.
+//
+// "RANDOM" MEANS DERIVED. The operator asked for "random subdued colors", but a tint
+// rolled at render time would differ between the phone and the browser and change on
+// every reload. Hashing the note's id is deterministic, identical on every surface,
+// costs no column and no migration, and a note keeps its colour for life — which is
+// what "random" actually meant here.
+//
+// THIS IS HALF A MIRRORED PAIR. `frontend/src/notes/colors.ts` computes the same hash
+// over the same key order, and the two must agree exactly or a note is one colour on
+// the phone and another in the browser. Same discipline as the checklist grammar's
+// three implementations, and the same reason: a value that disagrees across surfaces
+// is a bug you cannot unsee and cannot explain.
+//
+// NO COMPOSE IN THIS FILE, deliberately. It is the half of the pair that CAN be
+// pinned by a host-JVM test, and staying free of `androidx.compose` is what keeps
+// `DerivedTintTest` runnable in the Unit tests step rather than on an emulator. The
+// web side has no test runner at all, so this test is the only mechanical guard the
+// mirror gets — see the fixture comment in colors.ts.
+
+/**
+ * The tints a derived colour can land on: `NOTE_TINTS`' keys minus `default`, which
+ * is the white this exists to eliminate. `gray` stays — `bg-neutral-100` reads as a
+ * deliberate card against the board's `bg-neutral-50`, not as an absence.
+ *
+ * Order is load-bearing and matches `DERIVED_TINT_KEYS` in colors.ts. Reordering
+ * this list silently recolours every untagged note on one surface only.
+ */
+val DERIVED_TINT_KEYS: List<String> =
+    listOf("red", "orange", "yellow", "green", "teal", "blue", "purple", "pink", "gray")
+
+private const val FNV_OFFSET_BASIS = -0x7ee3623b // 0x811c9dc5 as a signed Int
+private const val FNV_PRIME = 0x01000193
+private const val BYTE_MASK = 0xFF
+private const val UNSIGNED_MASK = 0xFFFFFFFFL
+
+/**
+ * FNV-1a over the id's bytes, 32-bit.
+ *
+ * Chosen because both languages compute it identically in ten lines with no library.
+ * Explicitly NOT `String.hashCode()`: Kotlin's is specified but JS has no equivalent,
+ * and reimplementing Java's from memory in TypeScript is exactly how a mirror drifts.
+ *
+ * `and BYTE_MASK` is a no-op for the ASCII of a UUID, and is kept because it states
+ * the intent — this hashes BYTES, so the TypeScript side reading `charCodeAt(i) &
+ * 0xff` is the same function rather than a coincidence.
+ *
+ * Overflow is the point: Kotlin's `Int` wraps on multiply, which is what the web's
+ * `Math.imul` exists to reproduce.
+ */
+fun tintHash(id: String): Int {
+    var hash = FNV_OFFSET_BASIS
+    for (ch in id) {
+        hash = hash xor (ch.code and BYTE_MASK)
+        hash *= FNV_PRIME
+    }
+    return hash
+}
+
+/** The tint a note with no colour of its own wears. Stable for the life of the note. */
+fun derivedTint(id: String): String {
+    // Through Long to read the hash as unsigned. A signed remainder would be negative
+    // for half of all ids and index out of the list.
+    val index = (tintHash(id).toLong() and UNSIGNED_MASK) % DERIVED_TINT_KEYS.size
+    return DERIVED_TINT_KEYS[index.toInt()]
+}
+
+/**
+ * The colour key to actually paint a note with.
+ *
+ * An explicitly-picked colour still wins — the picker is on its way out (milestone
+ * 309 step 5) but it has not gone yet, and a note the operator coloured by hand
+ * changing under them would read as data loss.
+ *
+ * `known` is passed in rather than read from `NOTE_TINTS` so this file stays free of
+ * Compose and therefore testable; `noteTintFor` supplies the real set.
+ */
+fun resolvedNoteColor(id: String, color: String, known: Set<String>): String =
+    when {
+        color.isNotEmpty() && color != "default" && color in known -> color
+        // A draft carries DRAFT_ID (""), so there is no identity to derive from yet.
+        // Staying white until the note exists costs one colour change at save time;
+        // hashing the empty string instead would give EVERY draft the same tint and
+        // then change it anyway, which is two surprises where one will do.
+        id.isEmpty() -> "default"
+        else -> derivedTint(id)
+    }

android/app/src/main/java/com/fabledsword/thoughtsync/ui/NoteCard.kt

@@ -37,7 +37,7 @@ fun NoteCard(
     onToggleItem: (Int, Boolean) -> Unit,
 ) {
     val dark = isSystemInDarkTheme()
-    val tint = noteTint(note.color)
+    val tint = noteTintFor(note.id, note.color)
 
     Column(
         modifier =

android/app/src/main/java/com/fabledsword/thoughtsync/ui/NoteEditorScreen.kt

@@ -63,7 +63,7 @@ fun NoteEditorScreen(
     onAction: (EditorAction) -> Unit,
 ) {
     val dark = isSystemInDarkTheme()
-    val tint = noteTint(note.color)
+    val tint = noteTintFor(note.id, note.color)
 
     // Keyed by the SESSION, not by note.id: the editor is reused across notes, so it
     // needs a key — but a draft's id changes the moment it is first saved, and

android/app/src/main/java/com/fabledsword/thoughtsync/ui/NoteTint.kt

@@ -175,3 +175,15 @@ val NOTE_TINTS: Map<String, NoteTint> =
 @Composable
 @ReadOnlyComposable
 fun noteTint(key: String): NoteTint = NOTE_TINTS[key] ?: NOTE_TINTS.getValue("default")
+
+/**
+ * The tint for a NOTE, which is not the same thing as looking up its stored key: a
+ * note that has no colour of its own gets one derived from its id, so that a board
+ * of untagged notes reads as individual items rather than a wall of white.
+ *
+ * See `DerivedTint.kt` — the rule lives there, free of Compose, so it can be tested.
+ */
+@Composable
+@ReadOnlyComposable
+fun noteTintFor(id: String, color: String): NoteTint =
+    noteTint(resolvedNoteColor(id, color, NOTE_TINTS.keys))

android/app/src/test/java/com/fabledsword/thoughtsync/ui/DerivedTintTest.kt

@@ -0,0 +1,102 @@
+package com.fabledsword.thoughtsync.ui
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Test
+
+/**
+ * Pins the derived-tint rule against `frontend/src/notes/colors.ts`.
+ *
+ * These are not tests of Kotlin — they are the ONE mechanical guard the mirrored pair
+ * has. The web side is TypeScript with no test runner (its CI lane is `vue-tsc
+ * --noEmit` and nothing else), so if these values drift, nothing on that surface will
+ * say so and a note will simply be a different colour on the phone than in the
+ * browser. The same four ids and hashes are written into colors.ts as a comment;
+ * changing either side means changing both and re-checking here.
+ */
+class DerivedTintTest {
+    @Test
+    fun `hashes match the fixture shared with the web`() {
+        // Kotlin's Int is signed, so the two hashes above 0x7FFFFFFF are written as
+        // their negative literal. The unsigned value in the comment is what colors.ts
+        // records and what an implementation of FNV-1a will actually produce.
+        assertEquals(-0x41B8712F, tintHash("00000000-0000-0000-0000-000000000000")) // 0xbe478ed1
+        assertEquals(0x3D75CC01, tintHash("11111111-1111-1111-1111-111111111111"))
+        assertEquals(-0x0EF71AD0, tintHash("6ba7b810-9dad-11d1-80b4-00c04fd430c8")) // 0xf108e530
+        assertEquals(0x5B651540, tintHash("f47ac10b-58cc-4372-a567-0e02b2c3d479"))
+    }
+
+    @Test
+    fun `tints match the fixture shared with the web`() {
+        assertEquals("purple", derivedTint("00000000-0000-0000-0000-000000000000"))
+        assertEquals("blue", derivedTint("11111111-1111-1111-1111-111111111111"))
+        assertEquals("orange", derivedTint("6ba7b810-9dad-11d1-80b4-00c04fd430c8"))
+        assertEquals("orange", derivedTint("f47ac10b-58cc-4372-a567-0e02b2c3d479"))
+    }
+
+    /** Half of all 32-bit hashes are negative as Kotlin Ints; a signed remainder would
+     *  index out of the list for those. The bug this catches is a crash, not a wrong
+     *  colour, so it is worth more than one id's worth of coverage. */
+    @Test
+    fun `every tint is a real palette key, over many ids`() {
+        for (n in 0 until 2000) {
+            val tint = derivedTint("note-$n")
+            assertEquals(true, tint in DERIVED_TINT_KEYS)
+        }
+    }
+
+    @Test
+    fun `the derived palette excludes default`() {
+        assertEquals(false, "default" in DERIVED_TINT_KEYS)
+        assertEquals(9, DERIVED_TINT_KEYS.size)
+    }
+
+    /** The order IS the mapping — reordering silently recolours every untagged note
+     *  on one surface only. Written out longhand so a reorder fails here loudly. */
+    @Test
+    fun `key order matches colors ts`() {
+        assertEquals(
+            listOf("red", "orange", "yellow", "green", "teal", "blue", "purple", "pink", "gray"),
+            DERIVED_TINT_KEYS,
+        )
+    }
+
+    @Test
+    fun `an explicitly picked colour still wins`() {
+        val known = DERIVED_TINT_KEYS.toSet() + "default"
+        assertEquals("teal", resolvedNoteColor("any-id", "teal", known))
+    }
+
+    @Test
+    fun `an unknown colour key falls back to the derived tint`() {
+        val known = DERIVED_TINT_KEYS.toSet() + "default"
+        val id = "00000000-0000-0000-0000-000000000000"
+        assertEquals("purple", resolvedNoteColor(id, "chartreuse", known))
+    }
+
+    /** `default` is not a choice, it is the absence of one — so a note stored as
+     *  `default` gets a derived tint rather than staying white. That is the whole
+     *  point of the change. */
+    @Test
+    fun `a default colour is treated as no colour`() {
+        val known = DERIVED_TINT_KEYS.toSet() + "default"
+        val id = "11111111-1111-1111-1111-111111111111"
+        assertEquals("blue", resolvedNoteColor(id, "default", known))
+        assertNotEquals("default", resolvedNoteColor(id, "default", known))
+    }
+
+    /** A draft has no id yet. It must not be hashed — see the comment in DerivedTint. */
+    @Test
+    fun `a draft stays default until it has an id`() {
+        val known = DERIVED_TINT_KEYS.toSet() + "default"
+        assertEquals("default", resolvedNoteColor("", "", known))
+        assertEquals("default", resolvedNoteColor("", "default", known))
+    }
+
+    /** The reason the feature exists: two adjacent notes should not look identical. */
+    @Test
+    fun `the tint spreads across the palette`() {
+        val seen = (0 until 500).map { derivedTint("spread-$it") }.toSet()
+        assertEquals(DERIVED_TINT_KEYS.size, seen.size)
+    }
+}

frontend/src/components/NoteCard.vue

@@ -7,6 +7,7 @@ import {
   NOTE_COLOR_KEYS,
   NOTE_COLOR_LABELS,
   NOTE_SWATCH_CLASSES,
+  resolveNoteColor,
   type NoteColor,
 } from "../notes/colors";
 import type { Note } from "../stores/notes";
@@ -189,8 +190,10 @@ async function snoozeReminder(minutes: number): Promise<void> {
   emit("reminder-changed");
 }
 
-function cardClass(color: NoteColor): string {
+// Takes the note, not its colour: a note with no colour of its own gets one
-  return NOTE_CARD_CLASSES[color] ?? NOTE_CARD_CLASSES.default;
+// derived from its id, so the card can no longer be painted from a single field.
+function cardClass(note: Note): string {
+  return NOTE_CARD_CLASSES[resolveNoteColor(note)] ?? NOTE_CARD_CLASSES.default;
 }
 
 function labelChip(color: string): string {
@@ -225,7 +228,7 @@ onBeforeUnmount(() => document.removeEventListener("mousedown", onDocMousedown))
     ref="root"
     class="group relative mb-4 break-inside-avoid rounded-xl border p-3 shadow-sm transition hover:shadow-md"
     :class="[
-      cardClass(note.color),
+      cardClass(note),
       dragging ? 'opacity-40' : '',
       dragOver
         ? 'scale-[1.02] shadow-lg ring-2 ring-brand ring-offset-2 ring-offset-white dark:ring-offset-neutral-950'

frontend/src/notes/colors.ts

@@ -84,3 +84,87 @@ export const NOTE_COLOR_LABELS: Record<NoteColor, string> = {
   pink: "Pink",
   gray: "Gray",
 };
+
+// ---------------------------------------------------------------------------
+// Derived tints — the colour a note has when nothing chose one for it.
+//
+// A board of `default` notes is a wall of white rectangles and the eye gets no
+// help telling one from the next. Every note now carries some tint; this is where
+// an untagged one gets it.
+//
+// "RANDOM" MEANS DERIVED. The operator asked for "random subdued colors", but a
+// tint rolled at render time would differ between the phone and the browser and
+// change on every reload. Hashing the note's id is deterministic, identical on
+// every surface, costs no column and no migration, and a note keeps its colour
+// for life — which is what "random" actually meant here.
+//
+// THIS IS HALF A MIRRORED PAIR. `android/.../ui/NoteTint.kt` computes the same
+// hash over the same key order, and the two must agree exactly or a note is one
+// colour on the phone and another in the browser. Same discipline as the
+// checklist grammar's three implementations, and the same reason: a value that
+// disagrees across surfaces is a bug you cannot unsee and cannot explain.
+//
+// The Kotlin side has a unit test pinning the fixture below. THIS SIDE HAS NO
+// MECHANICAL GUARD — the frontend has no test runner, only `vue-tsc --noEmit`.
+// If you change anything here, check it against the fixture by hand.
+
+/** The tints a derived colour can land on: the palette minus `default`, which is
+ *  the white this exists to eliminate. `gray` stays — `bg-neutral-100` reads as a
+ *  deliberate card against the board's `bg-neutral-50`, not as an absence. */
+export const DERIVED_TINT_KEYS: readonly NoteColor[] = NOTE_COLOR_KEYS.filter(
+  (key) => key !== "default",
+);
+
+/**
+ * FNV-1a over the id's bytes, 32-bit.
+ *
+ * Chosen because both languages compute it identically in ten lines with no
+ * library. Explicitly NOT `String.hashCode()`: Kotlin's is specified but JS has
+ * no equivalent, and reimplementing Java's from memory in TypeScript is exactly
+ * how a mirror drifts.
+ *
+ * `& 0xff` is a no-op for the ASCII of a UUID, and is kept because it states the
+ * intent — this hashes BYTES, so the Kotlin side reading `id[i].code and 0xFF`
+ * is the same function rather than a coincidence.
+ */
+export function tintHash(id: string): number {
+  let hash = 0x811c9dc5;
+  for (let i = 0; i < id.length; i++) {
+    hash ^= id.charCodeAt(i) & 0xff;
+    // Math.imul, not `*`: JS numbers are doubles and a 32-bit overflow would be
+    // silently kept as precision instead of wrapping the way Kotlin's Int does.
+    hash = Math.imul(hash, 0x01000193) >>> 0;
+  }
+  return hash >>> 0;
+}
+
+/** The tint a note with no colour of its own wears. Stable for the life of the note. */
+export function derivedTint(id: string): NoteColor {
+  return DERIVED_TINT_KEYS[tintHash(id) % DERIVED_TINT_KEYS.length];
+}
+
+/**
+ * The colour to actually paint a note.
+ *
+ * An explicitly-picked colour still wins — the picker is on its way out
+ * (milestone 309 step 5) but it has not gone yet, and a note the operator
+ * coloured by hand changing under them would read as data loss.
+ */
+export function resolveNoteColor(note: { id: string; color?: string | null }): NoteColor {
+  const picked = note.color as NoteColor | undefined | null;
+  if (picked && picked !== "default" && picked in NOTE_CARD_CLASSES) return picked;
+  // No id yet means an unsaved draft: nothing to derive from. Staying white until the
+  // note exists costs one colour change at save time; hashing the empty string would
+  // give every draft the same tint and then change it anyway.
+  if (!note.id) return "default";
+  return derivedTint(note.id);
+}
+
+// Fixture — the same ids and expected keys the Kotlin test asserts. Kept here as
+// prose because there is nowhere on this side to assert it. If you change the hash
+// or the key order, these four must still hold on BOTH surfaces:
+//
+//   00000000-0000-0000-0000-000000000000  0xbe478ed1  purple
+//   11111111-1111-1111-1111-111111111111  0x3d75cc01  blue
+//   6ba7b810-9dad-11d1-80b4-00c04fd430c8  0xf108e530  orange
+//   f47ac10b-58cc-4372-a567-0e02b2c3d479  0x5b651540  orange