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packages/foundation/data/pagination/src/offset/offset.md

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+# Offset Pagination
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+## Purpose
+
+The offset pagination module provides the computational building blocks for traditional page-number-based pagination. It handles request normalisation, metadata calculation, page-to-offset conversion, and in-memory array slicing. All functions are pure and synchronous except `estimateTotalCount`, which calls an adapter to sample the dataset.
+
+Offset pagination is the correct choice when total count is cheap to compute, pages are shallow (< a few thousand rows), or the UI must render absolute page numbers. For deep pagination on large datasets, prefer keyset or cursor pagination to avoid scanning skipped rows.
+
+## Features
+
+- `calculateOffsetMeta` — compute full metadata (page, limit, total, totalPages, hasNext, hasPrevious).
+- `createOffsetResponse` — assemble a typed `OffsetPaginatedResponse<T>` envelope.
+- `normalizeOffsetRequest` — clamp page/limit to configured defaults, default sortOrder.
+- `pageToOffset` — convert 1-based page number to a zero-based array/SQL offset.
+- `offsetToPage` — convert a zero-based offset back to a 1-based page number.
+- `getPageRange` — produce a windowed list of page numbers for UI navigation controls.
+- `paginateArray` — slice an in-memory array and return a full `OffsetPaginatedResponse<T>`.
+- `estimateTotalCount` — sample-based approximate total count via adapter.
+
+## Usage: Basic Example
+
+```typescript
+import { paginateArray } from "@suaveplan/pagination/offset";
+
+const items = Array.from({ length: 50 }, (_, i) => ({ id: i + 1 }));
+const page2 = paginateArray(items, 2, 10);
+// page2.data → items 11–20
+// page2.meta.page → 2
+// page2.meta.totalPages → 5
+// page2.meta.hasNext → true
+// page2.meta.hasPrevious → true
+```
+
+## Usage: Advanced Example
+
+```typescript
+import {
+  normalizeOffsetRequest,
+  getPageRange,
+  pageToOffset,
+  offsetToPage,
+} from "@suaveplan/pagination/offset";
+
+// Normalise untrusted input from a query string
+const req = normalizeOffsetRequest(
+  { page: -5, limit: 9999 },
+  { maxLimit: 100, defaultLimit: 20 }
+);
+// req.page → 1 (clamped), req.limit → 100 (clamped)
+
+// Render page buttons for a 300-item result set
+const pages = getPageRange(15, 30, 7);
+// → [12, 13, 14, 15, 16, 17, 18]
+
+// Convert offset 40 at limit 10 → page 5
+const page = offsetToPage(40, 10); // → 5
+const offset = pageToOffset(5, 10); // → 40
+```
+
+## API Reference
+
+### `calculateOffsetMeta(total, page, limit): OffsetPaginationMeta`
+
+| Parameter | Type     | Description                          |
+|-----------|----------|--------------------------------------|
+| `total`   | `number` | Total item count across all pages    |
+| `page`    | `number` | Current 1-based page number          |
+| `limit`   | `number` | Items per page                       |
+
+Returns `OffsetPaginationMeta` with `totalPages`, `hasNext`, and `hasPrevious`.
+
+### `createOffsetResponse<T>(data, total, page, limit): OffsetPaginatedResponse<T>`
+
+Wraps `data` and computed metadata into the standard response envelope.
+
+### `normalizeOffsetRequest(input, defaults?): OffsetPaginationRequest`
+
+Clamps `page` to `≥ 1` and `limit` to `[minLimit, maxLimit]`. Defaults: minLimit=1, maxLimit=100, defaultLimit=20, defaultPage=1.
+
+### `pageToOffset(page, limit): number`
+
+Returns `Math.max(0, (page - 1) * limit)`.
+
+### `offsetToPage(offset, limit): number`
+
+Returns `Math.floor(offset / limit) + 1`. Returns `1` if `limit ≤ 0`.
+
+### `getPageRange(currentPage, totalPages, windowSize?): number[]`
+
+Default `windowSize` is 5. The window is centred on `currentPage` and never exceeds `[1, totalPages]`.
+
+### `paginateArray<T>(items, page, limit): OffsetPaginatedResponse<T>`
+
+Slices `items[offset..offset+limit]` and builds the full response with computed metadata.
+
+### `estimateTotalCount(adapter, opts): Promise<number>`
+
+Samples up to `opts.sampleSize` (default 1000) items from the adapter and extrapolates. Returns `0` if the adapter reports zero total.
+
+## Implementation Notes
+
+`normalizeOffsetRequest` uses `Math.floor` to discard fractional page/limit values from untrusted API input. The `sortOrder` field defaults to `"desc"` because most read-heavy UIs show newest items first. `getPageRange` is intentionally view-agnostic — it returns page numbers only; rendering (ellipsis, prev/next buttons) belongs in the UI layer.