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Algorithm Overview
Coordinate Systems
Integration
Main Entry Point

CSS layout engine for OCaml (Flexbox, Grid, Block)

Install

dune-project

Dependency

github.com Readme Changelog Edit opam file Versions (1)

Authors

Thibaut Mattio

Maintainers

Thibaut Mattio

Sources

mosaic-0.1.0.tbz

sha256=9e4e90d17f9b2af1b07071fe425bc2c519c849c4f1d1ab73cde512be2d874849

sha512=06e9c4a741590942e81a27738d0b5c0413fafec8cf3b7dae047ad69f155e7b718aa4223818dc161b7d028efffcfd3365905e264d6fd31d453910ddfa91dcf9b9

doc/compute_grid/Compute_grid/Placement/index.html

Module `Compute_grid.Placement`Source

CSS Grid item placement algorithm.

Implements the 8.5 Grid Item Placement Algorithm from the CSS Grid specification, which determines the final position of each grid item within the implicit grid.

Reference: CSS Grid Spec §8.5

Algorithm Overview

The placement algorithm processes grid items in four phases:

Place items with definite positions in both axes
Place items with definite secondary axis positions
Implicit grid sizing (handled by earlier estimate)
Auto-place remaining items

The algorithm operates on a Cell_occupancy.t matrix that tracks which cells are occupied and dynamically expands the implicit grid as needed. It respects grid-auto-flow settings (row/column, dense/sparse) and resolves named grid lines via Named.t.

Coordinate Systems

Uses origin-zero coordinates where explicit grid tracks are numbered from 0. Negative implicit tracks have negative indices; positive implicit tracks extend beyond the explicit grid. See Grid_track_counts for coordinate system details.

Integration

Called by the grid layout algorithm after computing grid size estimates. Populates items list and cell_occupancy_matrix with placed items. The cell occupancy matrix tracks both placement state (definite vs. auto) and implicit grid growth.

Main Entry Point

Source

val place_grid_items : 
  (module Tree.LAYOUT_PARTIAL_TREE with type t = 'tree) ->
  cell_occupancy_matrix:Cell_occupancy.t ->
  items:Grid_item.t list Stdlib.ref ->
  tree:'tree ->
  parent_node:Tree.Node_id.t ->
  grid_auto_flow:Style.grid_auto_flow ->
  align_items:Style.align_items ->
  justify_items:Style.align_items ->
  named_line_resolver:Named.t ->
  unit

place_grid_items (module Tree) ~cell_occupancy_matrix ~items ~tree ~parent_node ~grid_auto_flow ~align_items ~justify_items ~named_line_resolver places all grid items into the grid.

Processes children of parent_node in four phases: definite placement, definite secondary axis, implicit grid expansion, and auto-placement. Mutates cell_occupancy_matrix to track occupancy and expand implicit grid. Accumulates placed items into items in reverse order then reverses at end.

The algorithm:

Phase 1: Items with both axes definite are placed directly
Phase 2: Items with secondary axis definite search along primary axis
Phase 3: Implicit grid sizing is already handled by pre-sizing
Phase 4: Remaining items are auto-placed using grid_auto_flow strategy

Dense auto-flow always searches from the grid start. Sparse auto-flow continues from the last placed item's position. Items are filtered to exclude display: none and position: absolute children.

Preconditions:

cell_occupancy_matrix pre-sized via grid size estimate
items is empty or will be cleared
named_line_resolver initialized with template and explicit counts