fix: use a better chunking function

cypress/e2e/chunks.cy.js

@@ -0,0 +1,58 @@
+const { getChunk, chunkify } = require('../../src/chunk')
+
+describe('getChunk', () => {
+  it('splits 4 specs across 4 machines', () => {
+    const specs = [1, 2, 3, 4]
+    let chunk = getChunk(specs, 4, 0)
+    expect(chunk, 'chunk 0').to.deep.equal([1])
+    chunk = getChunk(specs, 4, 1)
+    expect(chunk, 'chunk 1').to.deep.equal([2])
+    chunk = getChunk(specs, 4, 2)
+    expect(chunk, 'chunk 2').to.deep.equal([3])
+    chunk = getChunk(specs, 4, 3)
+    expect(chunk, 'chunk 3').to.deep.equal([4])
+  })
+
+  it('splits 5 specs across 4 machines', () => {
+    const specs = [1, 2, 3, 4, 5]
+    let chunk = getChunk(specs, 4, 0)
+    expect(chunk, 'chunk 0').to.deep.equal([1, 2])
+    chunk = getChunk(specs, 4, 1)
+    expect(chunk, 'chunk 1').to.deep.equal([3])
+    chunk = getChunk(specs, 4, 2)
+    expect(chunk, 'chunk 2').to.deep.equal([4])
+    chunk = getChunk(specs, 4, 3)
+    expect(chunk, 'chunk 3').to.deep.equal([5])
+  })
+
+  it('splits 1 spec across 2 machines', () => {
+    const specs = [1]
+    let chunk = getChunk(specs, 2, 0)
+    expect(chunk, 'chunk 0').to.deep.equal([1])
+    chunk = getChunk(specs, 2, 1)
+    expect(chunk, 'chunk 1').to.deep.equal(undefined)
+  })
+})
+
+describe('chunkify', () => {
+  it('splits 5 specs across 1 machine', () => {
+    const specs = [1, 2, 3, 4, 5]
+    const chunks = chunkify(specs, 1, true)
+    expect(chunks).to.deep.equal([[1, 2, 3, 4, 5]])
+  })
+
+  it('splits 5 specs across 2 machines', () => {
+    const specs = [1, 2, 3, 4, 5]
+    const chunks = chunkify(specs, 2, true)
+    expect(chunks).to.deep.equal([
+      [1, 2, 3],
+      [4, 5],
+    ])
+  })
+
+  it('splits 5 specs across 6 machines', () => {
+    const specs = [1, 2, 3, 4, 5]
+    const chunks = chunkify(specs, 6, true)
+    expect(chunks).to.deep.equal([[1], [2], [3], [4], [5]])
+  })
+})

src/chunk.js

@@ -0,0 +1,49 @@
+// https://stackoverflow.com/questions/8188548/splitting-a-js-array-into-n-arrays
+function chunkify(a, n, balanced) {
+  if (n < 2) return [a]
+
+  var len = a.length,
+    out = [],
+    i = 0,
+    size
+
+  if (len % n === 0) {
+    size = Math.floor(len / n)
+    while (i < len) {
+      out.push(a.slice(i, (i += size)))
+    }
+  } else if (balanced) {
+    while (i < len) {
+      size = Math.ceil((len - i) / n--)
+      out.push(a.slice(i, (i += size)))
+    }
+  } else {
+    n--
+    size = Math.floor(len / n)
+    if (len % size === 0) size--
+    while (i < size * n) {
+      out.push(a.slice(i, (i += size)))
+    }
+    out.push(a.slice(size * n))
+  }
+
+  return out
+}
+
+function getChunk(values, totalChunks, chunkIndex) {
+  // split all items into N chunks and take just a single chunk
+  if (totalChunks < 0) {
+    throw new Error('totalChunks must be >= 0')
+  }
+
+  if (chunkIndex < 0 || chunkIndex >= totalChunks) {
+    throw new Error(
+      `Invalid chunk index ${chunkIndex} vs all chunks ${totalChunks}`,
+    )
+  }
+
+  const chunks = chunkify(values, totalChunks, true)
+  return chunks[chunkIndex]
+}
+
+module.exports = { getChunk, chunkify }

src/index.js

@@ -2,27 +2,7 @@ const debug = require('debug')('cypress-split')
 const { getSpecs } = require('find-cypress-specs')
 const ghCore = require('@actions/core')
 const cTable = require('console.table')
-
-function getChunk(values, totalChunks, chunkIndex) {
-  debug('get chunk %o', { values, totalChunks, chunkIndex })
-
-  // split all items into N chunks and take just a single chunk
-  if (totalChunks < 0) {
-    throw new Error('totalChunks must be >= 0')
-  }
-
-  if (chunkIndex < 0 || chunkIndex >= totalChunks) {
-    throw new Error(
-      `Invalid chunk index ${chunkIndex} vs all chunks ${totalChunks}`,
-    )
-  }
-
-  const chunkSize = Math.ceil(values.length / totalChunks)
-  const chunkStart = chunkIndex * chunkSize
-  const chunkEnd = chunkStart + chunkSize
-  const chunk = values.slice(chunkStart, chunkEnd)
-  return chunk
-}
+const { getChunk } = require('./chunk')
 
 const label = 'cypress-split:'
 
@@ -104,6 +84,8 @@ function cypressSplit(on, config) {
     const splitN = Number(SPLIT)
     const splitIndex = Number(SPLIT_INDEX)
     console.log('%s split %d of %d', label, splitIndex, splitN)
+
+    debug('get chunk %o', { values, totalChunks, chunkIndex })
     const splitSpecs = getChunk(specs, splitN, splitIndex)
 
     const specRows = splitSpecs.map((specName, k) => {