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title	id	challengeType
Hofstadter Figure-Figure sequences	59622f89e4e137560018a40e	5

Description

These two sequences of positive integers are defined as:

$$R(1)=1\ ;\ S(1)=2 \\R(n)=R(n-1)+S(n-1), \quad n>1.$$

The sequence $S(n)$ is further defined as the sequence of positive integers not present in $R(n)$.

Sequence $R$ starts:

1, 3, 7, 12, 18, ...

Sequence $S$ starts:

2, 4, 5, 6, 8, ...

Task: Create two functions named ffr and ffs that when given n return R(n) or S(n) respectively.(Note that R(1) = 1 and S(1) = 2 to avoid off-by-one errors). No maximum value for n should be assumed. Sloane's A005228 and A030124. Wolfram MathWorld Wikipedia: Hofstadter Figure-Figure sequences.

Instructions

Tests

tests:
  - text: <code>ffr</code> is a function.
    testString: assert(typeof ffr === 'function', '<code>ffr</code> is a function.');
  - text: <code>ffs</code> is a function.
    testString: assert(typeof ffs === 'function', '<code>ffs</code> is a function.');
  - text: <code>ffr</code> should return integer.
    testString: assert(Number.isInteger(ffr(1)), '<code>ffr</code> should return integer.');
  - text: <code>ffs</code> should return integer.
    testString: assert(Number.isInteger(ffs(1)), '<code>ffs</code> should return integer.');
  - text: <code>ffr()</code> should return <code>69</code>
    testString: assert.equal(ffr(ffrParamRes[0][0]), ffrParamRes[0][1], '<code>ffr()</code> should return <code>69</code>');
  - text: <code>ffr()</code> should return <code>1509</code>
    testString: assert.equal(ffr(ffrParamRes[1][0]), ffrParamRes[1][1], '<code>ffr()</code> should return <code>1509</code>');
  - text: <code>ffr()</code> should return <code>5764</code>
    testString: assert.equal(ffr(ffrParamRes[2][0]), ffrParamRes[2][1], '<code>ffr()</code> should return <code>5764</code>');
  - text: <code>ffr()</code> should return <code>526334</code>
    testString: assert.equal(ffr(ffrParamRes[3][0]), ffrParamRes[3][1], '<code>ffr()</code> should return <code>526334</code>');
  - text: <code>ffs()</code> should return <code>14</code>
    testString: assert.equal(ffs(ffsParamRes[0][0]), ffsParamRes[0][1], '<code>ffs()</code> should return <code>14</code>');
  - text: <code>ffs()</code> should return <code>59</code>
    testString: assert.equal(ffs(ffsParamRes[1][0]), ffsParamRes[1][1], '<code>ffs()</code> should return <code>59</code>');
  - text: <code>ffs()</code> should return <code>112</code>
    testString: assert.equal(ffs(ffsParamRes[2][0]), ffsParamRes[2][1], '<code>ffs()</code> should return <code>112</code>');
  - text: <code>ffs()</code> should return <code>1041</code>
    testString: assert.equal(ffs(ffsParamRes[3][0]), ffsParamRes[3][1], '<code>ffs()</code> should return <code>1041</code>');

Challenge Seed

// noprotect
function ffr(n) {
  return n;
}

function ffs(n) {
  return n;
}

After Test

const ffrParamRes = [[10, 69], [50, 1509], [100, 5764], [1000, 526334]];
const ffsParamRes = [[10, 14], [50, 59], [100, 112], [1000, 1041]];

Solution

// noprotect
const R = [null, 1];
const S = [null, 2];

function extendSequences (n) {
  let current = Math.max(R[R.length - 1], S[S.length - 1]);
  let i;
  while (R.length <= n || S.length <= n) {
    i = Math.min(R.length, S.length) - 1;
    current += 1;
    if (current === R[i] + S[i]) {
      R.push(current);
    } else {
      S.push(current);
    }
  }
}

function ffr (n) {
  extendSequences(n);
  return R[n];
}

function ffs (n) {
  extendSequences(n);
  return S[n];
}

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