Module Compute_flexboxSource

Flexbox layout computation algorithm.

Implements the CSS Flexbox specification (https://www.w3.org/TR/css-flexbox-1/) following the W3C algorithm structure. This is a direct port of taffy's flexbox implementation.

Algorithm Overview

The flexbox algorithm proceeds in stages following the W3C spec numbering:

1. Initial Setup: Generate flex items from children, compute constants from container style.
2. Line Length Determination: Determine available space, compute flex base sizes and hypothetical sizes, determine container main size.
3. Main Size Determination: Collect items into lines, resolve flexible lengths.
4. Cross Size Determination: Determine hypothetical cross sizes, calculate baselines, compute line cross sizes, handle align-content stretch, determine used cross sizes.
5. Main-Axis Alignment: Distribute remaining free space using justify-content.
6. Cross-Axis Alignment: Resolve cross-axis auto margins, determine container cross size, align lines, align items within lines.
7. Final Positioning: Resolve insets, compute absolute positions, accumulate scroll offsets, handle hidden items.

Key Invariants

• flex_item.frozen determines whether an item's size is locked during flexible length resolution.
• flex_item.violation tracks deviation from target size; items are frozen when violations resolve.
• flex_line.items is a slice into the main flex_items array, avoiding allocations.
• algo_constants caches resolved style properties and container sizes updated during layout.
• Main axis direction (row/column) determines which dimension (width/height) is used for main/cross sizing.

Performance Notes

• Uses array-based iteration over flex items and lines for cache locality.
• Mutable fields in flex_item, flex_line, and algo_constants avoid allocations during iterative sizing.
• Percentage gaps are re-resolved after container main size is determined.
• Short-circuits in compute_flexbox_layout when size is fully determined and run mode is ComputeSize.