#include ...
#include ...
#define LAY_IMPLEMENTATION
#include "layout.h"

// LAY_IMPLEMENTATION needs to be defined in exactly one .c or .cpp file that
// includes layout.h. All other files should not define it.

#define LAY_IMPLEMENTATION
#include "layout.h"

// Let's pretend we're creating some kind of GUI with a master list on the
// left, and the content view on the right.

// We first need one of these
lay_context ctx;

// And we need to initialize it
lay_init_context(&ctx);

// The context will automatically resize its heap buffer to grow as needed
// during use. But we can avoid multiple reallocations by reserving as much
// space as we'll need up-front. Don't worry, lay_init_context doesn't do any
// allocations, so this is our first and only alloc.
lay_reserve_items_capacity(&ctx, 1024);

// Create our root item. Items are just 2D boxes.
lay_id root = lay_item(&ctx);

// Let's pretend we have a window in our game or OS of some known dimension.
// We'll want to explicitly set our root item to be that size.
lay_set_size_xy(&ctx, root, 1280, 720);

// Set our root item to arrange its children in a row, left-to-right, in the
// order they are inserted.
lay_set_contain(&ctx, root, LAY_ROW);

// Create the item for our master list.
lay_id master_list = lay_item(&ctx);
lay_insert(&ctx, root, master_list);
// Our master list has a specific fixed width, but we want it to fill all
// available vertical space.
lay_set_size_xy(&ctx, master_list, 400, 0);
// We set our item's behavior within its parent to desire filling up available
// vertical space.
lay_set_behave(&ctx, master_list, LAY_VFILL);
// And we set it so that it will lay out its children in a column,
// top-to-bottom, in the order they are inserted.
lay_set_contain(&ctx, master_list, LAY_COLUMN);

lay_id content_view = lay_item(&ctx);
lay_insert(&ctx, root, content_view);
// The content view just wants to fill up all of the remaining space, so we
// don't need to set any size on it.
//
// We could just set LAY_FILL here instead of bitwise-or'ing LAY_HFILL and
// LAY_VFILL, but I want to demonstrate that this is how you combine flags.
lay_set_behave(&ctx, content_view, LAY_HFILL | LAY_VFILL);

// Normally at this point, we would probably want to create items for our
// master list and our content view and insert them. This is just a dumb fake
// example, so let's move on to finishing up.

// Run the context -- this does all of the actual calculations.
lay_run_context(&ctx);

// Now we can get the calculated size of our items as 2D rectangles. The four
// components of the vector represent x and y of the top left corner, and then
// the width and height.
lay_vec4 master_list_rect = lay_get_rect(&ctx, master_list);
lay_vec4 content_view_rect = lay_get_rect(&ctx, content_view);

// master_list_rect  == {  0, 0, 400, 720}
// content_view_rect == {400, 0, 880, 720}

// If we're using an immediate-mode graphics library, we could draw our boxes
// with it now.
my_ui_library_draw_box_x_y_width_height(
    master_list_rect[0],
    master_list_rect[1],
    master_list_rect[2],
    master_list_rect[3]);

// You could also recursively go through the entire item hierarchy using
// lay_first_child and lay_next_sibling, or something like that.

// After you've used lay_run_context, the results should remain valid unless a
// reallocation occurs.
//
// However, while it's true that you could manually update the existing items
// in the context by using lay_set_size{_xy}, and then calling lay_run_context
// again, you might want to consider just rebuilding everything from scratch
// every frame. This is a lot easier to program than tedious fine-grained
// invalidation, and a context with thousands of items will probably still only
// take a handful of microseconds.
//
// There's no way to remove items -- once you create them and insert them,
// that's it. If we want to reset our context so that we can rebuild our layout
// tree from scratch, we use lay_reset_context:

lay_reset_context(&ctx);

// And now we could start over with creating the root item, inserting more
// items, etc. The reason we don't create a new context from scratch is that we
// want to reuse the buffer that was already allocated.

// But let's pretend we're shutting down our program -- we need to destroy our
// context.
lay_destroy_context(&ctx);

// The heap-allocated buffer is now freed. The context is now invalid for use
// until lay_init_context is called on it again.