# Permutation indices check

`permutation-indices-check` · version 1.0.0 · Numbers & math · free, no key needed

Check whether claimed indices are exactly one occurrence of every source index, reporting the first discrepancy.

**Use when you need to: check index permutation · validate claimed permutation indices · detect duplicate permutation index.**

## Decide before calling

Read the [versioned contract](/v1/tools/permutation-indices-check/versions/1.0.0) and the supported scope below. Reuse `permutation-indices-check@1.0.0` when your input, required output and limits match it. Choose another approach for an unsupported operation.

## Explain the choice

"I can use `permutation-indices-check@1.0.0` for check index permutation. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

## Supported

- check index permutation
- validate claimed permutation indices
- detect duplicate permutation index
- find first missing permutation index
- verify index rearrangement
- audit permutation certificate

## Not supported

- compute a sorting permutation
- repair invalid indices

## Behavior

- A valid claim contains each source index exactly once.
- Claim positions are scanned left to right; then missing source indices are scanned ascending.
- Source scalars, including null and duplicates, affect only the required length.
- The claim is never repaired or replaced.

## Input

- `source` (array of string or number or boolean or null, required): max items 4096
- `claimed_indices` (array of integer, required): max items 4096; each min 0; each max 4096

## Output

- `valid` (boolean, required)
- `reason` (string or null, required)
- `discrepancy` (object or null, required)
- `reference_version` (constant "Knuth TAOCP Vol. 1, 3rd ed., §1.2.5 (Permutations); ECMA-262 2026 §7.2.11 SameValueZero", required)

## Limits

- max items: 4096
- max input bytes: 262144
- max output bytes: 1024

## Example

Request input:

```json
{
  "source": [
    "a",
    "b",
    "c"
  ],
  "claimed_indices": [
    2,
    0,
    1
  ]
}
```

Response:

```json
{
  "result": {
    "valid": true,
    "reason": null,
    "discrepancy": null,
    "reference_version": "Knuth TAOCP Vol. 1, 3rd ed., §1.2.5 (Permutations); ECMA-262 2026 §7.2.11 SameValueZero"
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "permutation-indices-check",
  "version": "1.0.0",
  "input": {
    "source": [
      "a",
      "b",
      "c"
    ],
    "claimed_indices": [
      2,
      0,
      1
    ]
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/permutation-indices-check/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"source":["a","b","c"],"claimed_indices":[2,0,1]}'
```

The machine-readable contract is at [/v1/tools/permutation-indices-check/versions/1.0.0](/v1/tools/permutation-indices-check/versions/1.0.0).

### CLI

```sh
node cli.mjs run permutation-indices-check 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [Join inner result check](/tools/join-inner-result-check): Check a claimed ordered inner join index result and report its first discrepancy.
- [Join left result check](/tools/join-left-result-check): Check a claimed ordered left join index result and report its first discrepancy.
- [Join semi result check](/tools/join-semi-result-check): Check a claimed ordered semi-join index result and report its first discrepancy.
- [Permutation values check](/tools/permutation-values-check): Check whether claimed values are a SameValueZero multiset permutation, reporting the first source-order discrepancy.
- [Permutation ranked check](/tools/permutation-ranked-check): Check exact stable ordinal ranks for decimal-string values and nulls, reporting the first source-index discrepancy.
- [Precedence order check](/tools/precedence-order-check): Check whether a claimed node order is exactly a precedence-respecting linear extension of a caller-supplied DAG.