Adding a Window Operator

Audience: Pulse internals contributors adding a new WIN_* operator — a window-frame function that emits a per-row value derived from a sliding or anchored frame around the current record (WIN_LAG, WIN_LEAD, WIN_RANK, WIN_RUNNING_*, WIN_EWMA, …).

The recipe mirrors the aggregator recipe; the window-specific moving parts are the frame contract, the windows registry, and the streamability case.

1. Declare the type constant

Add the new constant to types/types.go and the slice returned by types.AllWindowTypes():

const (
    // ... existing constants ...
    WIN_PERCENT_RANK WindowType = "WIN_PERCENT_RANK"
)

func AllWindowTypes() []WindowType {
    return []WindowType{
        // ... existing entries, alphabetised ...
        WIN_PERCENT_RANK,
    }
}

2. Implement and register

Window operators live under processing/window/. Each file is one operator; register the factory in the package’s init() via the register(types.WIN_X, newX) call shape used by sibling files.

The frame semantics — anchored vs sliding, lookahead vs lookback — are part of the operator’s contract and ride in the operator’s struct parameters; the orchestrator’s window-pass invokes the implementation per row in declared order.

3. Declare streamability

Window operators that need a full forward / backward pass (lead-based, percent-rank style) are not streamable. Add the case in types/streamability.go:

func (t WindowType) Streamable() bool {
    switch t {
    // ...
    case WIN_PERCENT_RANK:
        return false
    }
}

Add the matching row to types/streamability_test.go so TestStreamability_WindowsKnown passes.

4. Capability declaration

Add a row to descriptor/capabilities_windows.go with the operator’s params, accepted field types, and streamable hint. TestManifestOperatorsComplete enforces a capability row per registered window operator.

5. Tests

Add tests in processing/window/<name>_test.go. Cover the empty-frame, single-row, null-bearing, and order-sensitive cases.

6. Update the window-operations skill

Add a section in skills/window-design.md covering the operator’s frame contract, parameter shape, and output column naming. The TestSkillsCoverAllWindowTypes gate enforces presence.

7. Update CLAUDE.md

Bump the registered-window count in CLAUDE.md’s “Skill Pack” section.

8. Run the gates

go test ./skills/ -run TestSkillsCoverAllWindowTypes
go test ./descriptor/ -run TestManifestOperatorsComplete
go test ./processing/window/...
go test ./types/ -run TestStreamability_WindowsKnown

The Update Demand row for windows covers all of these in one PR; see The Update Demand.