Detecting Moved Items in JSON Arrays

When an array element changes position without being modified, a naive diff algorithm reports it as a removal at the old index and an addition at the new index. Move detection is an advanced feature that recognizes this pattern and reports a single "move" operation instead.

The move detection problem:

// Before
{
  "steps": [
    { "id": "build", "name": "Build" },
    { "id": "test", "name": "Test" },
    { "id": "lint", "name": "Lint" },
    { "id": "deploy", "name": "Deploy" }
  ]
}

// After
{
  "steps": [
    { "id": "lint", "name": "Lint" },
    { "id": "build", "name": "Build" },
    { "id": "test", "name": "Test" },
    { "id": "deploy", "name": "Deploy" }
  ]
}

Without move detection (index-based diff):

• Index 0: changed from build to lint
• Index 1: changed from test to build
• Index 2: changed from lint to test

This is noisy and obscures the actual change: lint was moved from index 2 to index 0.

With move detection (identity-based diff using id):

• lint: moved from index 2 to index 0

This accurately represents the single reordering operation that occurred.

How move detection works:

1. Identity matching: The algorithm first matches elements across both arrays using a unique identifier (like id). This establishes which elements are "the same" regardless of position.
2. Position comparison: For matched elements, their indices are compared. Elements at different indices are candidates for move operations.
3. LCS optimization: The Longest Common Subsequence of matched elements determines which elements stayed in place and which moved. Elements not in the LCS are the ones that moved.

RFC 6902 JSON Patch:

The JSON Patch standard includes a "move" operation:

{ "op": "move", "from": "/steps/2", "path": "/steps/0" }

This is more concise and efficient than separate remove+add operations, especially for large arrays.

When move detection matters:

• CI/CD pipeline steps: Reordering build steps is a common change that should be clearly visible.
• Priority lists: Tasks or queue items reordered by priority.
• Column layouts: UI configuration where the order of displayed columns changes.