Expand the step-up probe by the requested movement

index.js

@@ -237,7 +237,7 @@ function Physics (mcData, world) {
 
       const BB1 = oldBB.clone()
       const BB2 = oldBB.clone()
-      const BB_XZ = BB1.clone().extend(dx, 0, dz)
+      const BB_XZ = BB1.clone().extend(oldVelX, 0, oldVelZ)
 
       let dy1 = dy
       let dy2 = dy

test/step.test.js

@@ -0,0 +1,111 @@
+/* eslint-env mocha */
+
+const { Physics, PlayerState } = require('prismarine-physics')
+const { Vec3 } = require('vec3')
+const expect = require('expect')
+
+// A player's 0.6 step height clears a one-sixteenth-tall block such as a
+// carpet. https://www.mcpk.wiki/wiki/Stepping
+//
+// Under a two-high ceiling that step was refused when the player was already
+// flush against the block, and allowed when it was not, which is what these
+// cases pin down.
+
+const version = '1.21'
+const mcData = require('minecraft-data')(version)
+const Block = require('prismarine-block')(version)
+
+const STONE = mcData.blocksByName.stone.defaultState
+const CARPET = mcData.blocksByName.moss_carpet.defaultState
+const AIR = mcData.blocksByName.air.defaultState
+
+/**
+ * Floor everywhere at y=60. West of the player, at x=-1, a carpet on that floor
+ * under a ceiling at y=63, which leaves the destination two cells high. The
+ * player starts flush against the carpet's east face with nothing overhead,
+ * unless `ceilingOverPlayer` closes its own cell in the same way.
+ */
+function corridor (ceilingOverPlayer) {
+  return {
+    getBlock: (pos) => {
+      let stateId = AIR
+      if (pos.y === 60) stateId = STONE
+      else if (pos.y === 61 && pos.x === -1) stateId = CARPET
+      else if (pos.y === 63 && pos.x === -1) stateId = STONE
+      else if (pos.y === 63 && pos.x === 0 && ceilingOverPlayer) stateId = STONE
+      const block = Block.fromStateId(stateId, 0)
+      block.position = pos
+      return block
+    }
+  }
+}
+
+/** Walk west, into the carpet, for `ticks` ticks. Yaw of PI/2 is -x. */
+function walkWest (world, ticks) {
+  const player = {
+    entity: {
+      // x=0.3 puts the player's west face on 0.0, flush with the carpet.
+      position: new Vec3(0.3, 61, 0.5),
+      velocity: new Vec3(0, 0, 0),
+      onGround: true,
+      isInWater: false,
+      isInLava: false,
+      isInWeb: false,
+      isCollidedHorizontally: false,
+      isCollidedVertically: false,
+      elytraFlying: false,
+      yaw: Math.PI / 2,
+      pitch: 0,
+      effects: {}
+    },
+    jumpTicks: 0,
+    jumpQueued: false,
+    fireworkRocketDuration: 0,
+    version,
+    inventory: { slots: [] }
+  }
+  const controls = {
+    forward: true,
+    back: false,
+    left: false,
+    right: false,
+    jump: false,
+    sprint: false,
+    sneak: false
+  }
+  const physics = Physics(mcData, world)
+  const state = new PlayerState(player, controls)
+  for (let tick = 0; tick < ticks; tick++) {
+    physics.simulatePlayer(state, world).apply(player)
+  }
+  return player.entity.position
+}
+
+describe('Stepping onto a thin block under a low ceiling', () => {
+  it('the fixture really is a one-sixteenth block', () => {
+    const carpet = Block.fromStateId(CARPET, 0)
+    const height = Math.max(...carpet.shapes.map(shape => shape[4]))
+
+    expect(height).toBeLessThan(0.1)
+    expect(height).toBeGreaterThan(0)
+  })
+
+  it('steps on to the carpet from a standstill flush against it', () => {
+    const position = walkWest(corridor(false), 10)
+
+    // Crossed into the carpet's cell, standing on top of it rather than in it.
+    expect(position.x).toBeLessThan(0)
+    expect(position.y).toBeGreaterThan(61)
+  })
+
+  it('does not depend on what stands over the player', () => {
+    // The same move, differing only in a block above the player's own head.
+    // That block cannot help the player walk forward, so both must travel.
+    const open = walkWest(corridor(false), 10)
+    const covered = walkWest(corridor(true), 10)
+
+    expect(open.x).toBeLessThan(0)
+    expect(covered.x).toBeLessThan(0)
+    expect(Math.abs(open.x - covered.x)).toBeLessThan(0.05)
+  })
+})