check-sequence-goldilocks.js

//see if sequence of nums is in increasing order, but seq is allowed to decrease once.

// The Goldilocks solution
function checkSequence(arr) {
  const maxOutOfSequencePermitted = 1
  let countOfOutOfSequence = 0
  for (let i = 1; i < arr.length; i += 1) {
    if (arr[i] < arr[i - 1]) {
      countOfOutOfSequence += 1
    }
    if (countOfOutOfSequence > maxOutOfSequencePermitted) {
      return false
    }
  }
  return true
}

function logSuccess(fn) {
  return arr => console.log(fn(arr) ? 'success' : 'failure')
}

logSuccess(checkSequence)([0, 1, 2, 3, 5, 4, 5, 7])

check-sequence-verbose.js

//see if sequence of nums is in increasing order, but seq is allowed to decrease once.
//some shared stuff first:

// Easier to read. (If not overly verbose)
function checkSequence(arr) {
  let countOfOutOfSequence = 0
  for (let i = 1; i < arr.length; i += 1) {
    if (isOutOfSequence(i)) {
      countOfOutOfSequence += 1
    }
    if (hasTooManyOutOfSequence()) {
      return false
    }
  }
  return true

  function isOutOfSequence(i) {
    return arr[i] < arr[i - 1]
  }

  function hasTooManyOutOfSequence() {
    const maxAllowableOutOfSequence = 1
    return countOfOutOfSequence > maxAllowableOutOfSequence
  }
}

function logSuccess(fn) {
  return arr => console.log(fn(arr) ? 'success' : 'failure')
}

logSuccess(checkSequence)([0, 1, 2, 3, 5, 4, 5, 7])

check-sequence.js

//see if sequence of nums is in increasing order, but seq is allowed to decrease once.
//some shared stuff first:

// Abusing the ternary
function checkSequence(arr) {
  return (
    arr.reduce(
      (decreaseCount, num, index) =>
        index === 0
          ? 0
          : num < arr[index - 1]
          ? decreaseCount + 1
          : decreaseCount,
      0
    ) <= 1
  )
}

function logSuccess(fn) {
  return arr => console.log(fn(arr) ? 'success' : 'failure')
}

logSuccess(checkSequence)([0, 1, 2, 3, 5, 4, 5, 7])

tests.js

/* 
  The following code tests checkSequence:
*/
const tests = [
  {
    name: '2 elements',
    testFn: () => {
      const arr = [0, 1]
      assertEquals(checkSequence(arr), true)
    }
  },
  {
    name: 'decreases twice',
    testFn: () => {
      const arr = [0, 1, 0, 1, 0]
      assertEquals(checkSequence(arr), false)
    }
  },
  {
    name: 'decreases once',
    testFn: () => {
      const arr = [0, 1, 2, 3, 5, 4, 5, 7]
      assertEquals(checkSequence(arr), true)
    }
  }
]

function assertEquals(actual, expected) {
  if (actual !== expected) {
    throw new Error(`Expected ${actual} to equal ${expected}.`)
  }
}

function runTest(testName, testFn) {
  try {
    testFn()
    console.log(`Passed test "${testName}".\n`)
  } catch (err) {
    console.log(`Failed test "${testName}":\n\n`)
    console.log(err)
    console.log('\n\n')
  }
}

tests.forEach(({ name, testFn }) => runTest(name, testFn))