Add falling pixel snow over the landing mountain

src/tui-opentui/mark-anim.test.ts

@@ -2,14 +2,20 @@ import { describe, expect, test } from "bun:test"
 
 import {
   MARK_PERIOD_SECONDS,
+  SNOW_CHAR,
   markFrame,
   markText,
   renderMark,
   smooth,
 } from "./mark-anim"
-import { MARK_COLS, MARK_COVERAGE, MARK_LARGE, MARK_ROWS } from "./mark-shape"
+import { MARK_COLS, MARK_LARGE, MARK_ROWS, MARK_SMALL } from "./mark-shape"
 import { UI } from "./theme"
 
+const MOUNTAIN_CHARS = "▁▂▃▄▅▆▇█"
+
+const isMountain = (char: string): boolean => MOUNTAIN_CHARS.includes(char)
+const isSnow = (char: string): boolean => char === SNOW_CHAR
+
 describe("smooth", () => {
   test("clamps outside [0, 1] and eases inside it", () => {
     expect(smooth(-3)).toBe(0)
@@ -62,20 +68,36 @@ describe("markFrame", () => {
 })
 
 describe("renderMark", () => {
-  const emptyCells = (grid: readonly (readonly { char: string }[])[]): number =>
-    grid.flat().filter((cell) => cell.char === " ").length
+  /** Mountain-block weight only — snow must not pollute silhouette metrics. */
+  const mountainWeight = (
+    grid: readonly (readonly { char: string }[])[],
+  ): number =>
+    grid
+      .flat()
+      .reduce((sum, cell) => sum + Math.max(0, MOUNTAIN_CHARS.indexOf(cell.char) + 1), 0)
+
+  const mountainCells = (
+    grid: readonly (readonly { char: string }[])[],
+  ): number => grid.flat().filter((cell) => isMountain(cell.char)).length
 
   test("is the mark's cell dimensions", () => {
     const grid = renderMark({ nowMs: 0, still: true })
     expect(grid).toHaveLength(MARK_ROWS)
     for (const row of grid) expect(row).toHaveLength(MARK_COLS)
   })
 
-  test("never paints outside the silhouette", () => {
-    const grid = renderMark({ nowMs: 2000, still: false })
+  test("sky is empty or snow; mountain cells never hold snow", () => {
+    const grid = renderMark({ nowMs: 2000, still: false, grid: MARK_LARGE })
     grid.forEach((row, y) => {
       row.forEach((cell, x) => {
-        if ((MARK_COVERAGE[y]?.[x] ?? 0) === 0) expect(cell.char).toBe(" ")
+        const coverage = MARK_LARGE.coverage[y]?.[x] ?? 0
+        if (coverage === 0) {
+          expect(cell.char === " " || isSnow(cell.char)).toBe(true)
+          if (isSnow(cell.char)) expect(cell.fg).toBe(UI.textFaint)
+        } else if (isMountain(cell.char)) {
+          expect(cell.fg).toBe(UI.action)
+          expect(isSnow(cell.char)).toBe(false)
+        }
       })
     })
   })
@@ -84,17 +106,19 @@ describe("renderMark", () => {
     const grid = renderMark({ nowMs: 0, still: true, grid: MARK_LARGE })
     grid.forEach((row, y) => {
       row.forEach((cell, x) => {
-        expect(" ▁▂▃▄▅▆▇█").toContain(cell.char)
+        // Still mode has no snow — only space or mountain blocks.
+        expect(` ${MOUNTAIN_CHARS}`).toContain(cell.char)
         if ((MARK_LARGE.coverage[y]?.[x] ?? 0) === 1) expect(cell.char).toBe("█")
       })
     })
   })
 
-  test("the still frame is clock-independent", () => {
+  test("the still frame is clock-independent and has no snow", () => {
     const a = markText(renderMark({ nowMs: 0, still: true }))
     const b = markText(renderMark({ nowMs: 987_654, still: true }))
     expect(b).toBe(a)
     expect(a.replace(/[\s\n]/g, "").length).toBeGreaterThan(0)
+    expect(a.includes(SNOW_CHAR)).toBe(false)
   })
 
   test("the animated frame advances with the injected clock", () => {
@@ -105,14 +129,14 @@ describe("renderMark", () => {
   })
 
   test("the outline reveals left to right", () => {
-    // Early in the draw phase only the leftmost columns may be lit.
+    // Early in the draw phase only the leftmost mountain columns may be lit.
     const grid = renderMark({ nowMs: 0.06 * MARK_PERIOD_SECONDS * 1000, still: false })
     const lit = grid.flatMap((row) =>
-      row.flatMap((cell, col) => (cell.char === " " ? [] : [col])),
+      row.flatMap((cell, col) => (isMountain(cell.char) ? [col] : [])),
     )
     expect(Math.max(...lit, -1)).toBeLessThan(MARK_COLS)
     const full = renderMark({ nowMs: 0.4 * MARK_PERIOD_SECONDS * 1000, still: false })
-    expect(emptyCells(grid)).toBeGreaterThan(emptyCells(full))
+    expect(mountainCells(full)).toBeGreaterThan(mountainCells(grid))
   })
 
   test("the fade thins the mark out toward empty", () => {
@@ -121,12 +145,10 @@ describe("renderMark", () => {
       nowMs: 0.995 * MARK_PERIOD_SECONDS * 1000,
       still: false,
     })
-    expect(emptyCells(fading)).toBeGreaterThan(emptyCells(held))
+    expect(mountainCells(held)).toBeGreaterThan(mountainCells(fading))
   })
 
   test("filling makes the mark denser than its outline alone", () => {
-    const weight = (grid: readonly (readonly { char: string }[])[]): number =>
-      grid.flat().reduce((sum, cell) => sum + " ▁▂▃▄▅▆▇█".indexOf(cell.char), 0)
     const outlineOnly = renderMark({
       nowMs: 0.42 * MARK_PERIOD_SECONDS * 1000,
       still: false,
@@ -135,6 +157,65 @@ describe("renderMark", () => {
       nowMs: 0.8 * MARK_PERIOD_SECONDS * 1000,
       still: false,
     })
-    expect(weight(filled)).toBeGreaterThan(weight(outlineOnly))
+    expect(mountainWeight(filled)).toBeGreaterThan(mountainWeight(outlineOnly))
+  })
+
+  test("snow drifts over time without overwriting the silhouette", () => {
+    // Sample across several seconds so flakes advance even at a slow fall rate.
+    const times = [0, 1500, 3000, 4500, 6000, 7500]
+    const snowSets = times.map((nowMs) => {
+      const grid = renderMark({ nowMs, still: false, grid: MARK_LARGE })
+      const snow: string[] = []
+      grid.forEach((row, y) => {
+        row.forEach((cell, x) => {
+          if (isSnow(cell.char)) {
+            snow.push(`${y},${x}`)
+            // Flakes live only in sky cells — never on mountain coverage.
+            expect(MARK_LARGE.coverage[y]?.[x] ?? 0).toBe(0)
+          }
+        })
+      })
+      return snow.join("|")
+    })
+
+    const withSnow = snowSets.filter((s) => s.length > 0)
+    expect(withSnow.length).toBeGreaterThan(1)
+    expect(new Set(withSnow).size).toBeGreaterThan(1)
+
+    // During the full-hold phase the ridgeline dominates the flake field.
+    const held = renderMark({
+      nowMs: 0.82 * MARK_PERIOD_SECONDS * 1000,
+      still: false,
+      grid: MARK_LARGE,
+    })
+    let flakes = 0
+    let mountains = 0
+    held.forEach((row, y) => {
+      row.forEach((cell, x) => {
+        if (isSnow(cell.char)) {
+          flakes += 1
+          expect(MARK_LARGE.coverage[y]?.[x] ?? 0).toBe(0)
+        }
+        if (isMountain(cell.char)) mountains += 1
+      })
+    })
+    expect(mountains).toBeGreaterThan(20)
+    expect(mountains).toBeGreaterThan(flakes)
+  })
+
+  test("still mode freezes the mark with no snow motion", () => {
+    const a = renderMark({ nowMs: 0, still: true, grid: MARK_SMALL })
+    const b = renderMark({ nowMs: 50_000, still: true, grid: MARK_SMALL })
+    expect(markText(b)).toBe(markText(a))
+    expect(a.flat().some((cell) => isSnow(cell.char))).toBe(false)
+  })
+
+  test("snow drops out during the fade-out phase, matching the mark", () => {
+    const fading = renderMark({
+      nowMs: 0.995 * MARK_PERIOD_SECONDS * 1000,
+      still: false,
+      grid: MARK_LARGE,
+    })
+    expect(fading.flat().some((cell) => isSnow(cell.char))).toBe(false)
   })
 })

src/tui-opentui/mark-anim.ts

@@ -7,6 +7,11 @@
  * wave; at hero size a terminal renders that as visible noise rather than as
  * shimmer, so the terminal mark is opaque instead.
  *
+ * Over the sky (zero-coverage cells) a sparse field of pixel snow falls on the
+ * same injected clock. Density and speed stay low so the ridgeline keeps its
+ * silhouette; `still` (idle or reduced motion) freezes the mark and drops the
+ * snow entirely. Mountain cells always win over flakes.
+ *
  * Everything here is pure and clock-injected: `nowMs` is the only time source,
  * so the caller's existing 250 ms status tick drives the animation and tests
  * drive it deterministically. There is no timer in this module.
@@ -70,6 +75,18 @@ const EIGHTHS = ["▁", "▂", "▃", "▄", "▅", "▆", "▇", "█"] as cons
  */
 const FILL_GAMMA = 0.6
 
+/** One snowflake pixel. Exported so tests can distinguish sky from mountain. */
+export const SNOW_CHAR = "·"
+
+/**
+ * Fraction of columns that host a flake. Kept low so the sky reads as empty
+ * with occasional drift rather than a storm.
+ */
+const SNOW_COLUMN_FRACTION = 0.18
+
+/** Baseline rows-per-second fall rate. Slow enough to feel like drift. */
+const SNOW_FALL_SPEED = 0.55
+
 export type MarkCell = {
   readonly char: string
   readonly fg: string
@@ -83,6 +100,35 @@ export type MarkInput = {
   readonly grid?: MarkGrid
 }
 
+/**
+ * Stable unit hash in [0, 1) from integer seeds. Pure and clock-independent so
+ * flake columns and phases never jitter between frames.
+ */
+function unitHash(a: number, b = 0): number {
+  const n = Math.imul(a + 1, 374761393) ^ Math.imul(b + 1, 668265263)
+  const x = Math.imul(n ^ (n >>> 13), 1274126177)
+  return ((x >>> 0) % 10_000) / 10_000
+}
+
+/**
+ * Whether a sky cell at (row, col) holds a flake at `seconds`. Sparse columns
+ * only; each active column carries one flake with a private phase and a slight
+ * speed variation so the field does not march as a rigid lattice.
+ */
+function snowflakeAt(
+  row: number,
+  col: number,
+  seconds: number,
+  rows: number,
+): boolean {
+  if (rows <= 0) return false
+  if (unitHash(col, 1) > SNOW_COLUMN_FRACTION) return false
+  const phase = unitHash(col, 2) * rows
+  const speed = SNOW_FALL_SPEED * (0.75 + unitHash(col, 3) * 0.5)
+  const wrapped = (((seconds * speed + phase) % rows) + rows) % rows
+  return Math.floor(wrapped) === row
+}
+
 /**
  * Composite one frame into a row-major cell grid.
  *
@@ -91,6 +137,9 @@ export type MarkInput = {
  * slopes instead of staircasing. No dither texture survives inside the shape —
  * the mark is a mountain, and a mountain is opaque.
  *
+ * Sky cells (zero coverage) may hold a single falling snow pixel. Flakes never
+ * overwrite mountain coverage, and `still` suppresses them entirely.
+ *
  * `alpha` has no terminal equivalent, so it scales the block height instead:
  * the mark sinks toward empty rather than blending to black.
  */
@@ -100,6 +149,9 @@ export function renderMark(input: MarkInput): readonly (readonly MarkCell[])[] {
   const { drawProg, fillProg, alpha } = markFrame(seconds, input.still)
   const revealed = drawProg * shape.cols
   const fillLine = shape.rows * (1 - fillProg)
+  // Fade out drops the snow too so the decoration doesn't outlast the mark
+  // it drifts over.
+  const snowOn = !input.still && alpha === 1
 
   const grid: MarkCell[][] = []
   for (let row = 0; row < shape.rows; row++) {
@@ -110,7 +162,17 @@ export function renderMark(input: MarkInput): readonly (readonly MarkCell[])[] {
       const coverage = shape.coverage[row]?.[col] ?? 0
       const reveal = clamp01(revealed - col)
       if (coverage === 0 || reveal === 0) {
-        cells.push({ char: " ", fg: UI.action })
+        // Snow only in true sky. Unrevealed mountain cells stay empty so the
+        // left-to-right draw still reads as a clean silhouette edge.
+        if (
+          snowOn &&
+          coverage === 0 &&
+          snowflakeAt(row, col, seconds, shape.rows)
+        ) {
+          cells.push({ char: SNOW_CHAR, fg: UI.textFaint })
+        } else {
+          cells.push({ char: " ", fg: UI.action })
+        }
         continue
       }
       // The outline states the shape at its true coverage; filling lifts it