<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\DB;
use Illuminate\Support\Facades\Schema;

return new class extends Migration
{
    public function up(): void
    {
        // assignments is PARTITIONED BY HASH (experiment_id) into 16 children.
        // PK is composite (experiment_id, unit_id) — sticky lookup is a
        // single index seek into the correct partition. Per-partition
        // secondary index on (unit_id) supports GET /v1/assignments/:unit_id;
        // that endpoint scans all 16 partitions but each is an index seek,
        // not a heap scan. Documented in docs/architecture/scaling.
        //
        // Laravel's schema builder doesn't support PARTITION BY directly, so
        // we run raw SQL.

        DB::statement(<<<'SQL'
            CREATE TABLE assignments (
                id              CHAR(26)        NOT NULL,
                experiment_id   CHAR(26)        NOT NULL,
                unit_id         VARCHAR(256)    NOT NULL,
                variant_id      CHAR(26)        NOT NULL,
                assignment_reason VARCHAR(32)   NOT NULL DEFAULT 'bucketed',
                assigned_at     TIMESTAMPTZ     NOT NULL DEFAULT NOW(),
                created_at      TIMESTAMPTZ     NOT NULL DEFAULT NOW(),
                updated_at      TIMESTAMPTZ     NOT NULL DEFAULT NOW(),
                PRIMARY KEY (experiment_id, unit_id)
            ) PARTITION BY HASH (experiment_id);
        SQL);

        // Create 16 hash partitions
        for ($i = 0; $i < 16; $i++) {
            DB::statement(sprintf(
                'CREATE TABLE assignments_p%d PARTITION OF assignments FOR VALUES WITH (MODULUS 16, REMAINDER %d);',
                $i,
                $i
            ));
            DB::statement(sprintf(
                'CREATE INDEX assignments_p%d_unit_idx ON assignments_p%d (unit_id);',
                $i,
                $i
            ));
        }

        // Mutex group claim — written on first call into any experiment in
        // the group; SELECT FOR UPDATE on read to handle simultaneous-call
        // race. Once written, never updated — INSERT-only at the DB grant
        // level (defense in depth).
        Schema::create('mutex_group_assignments', function (Blueprint $table) {
            $table->char('mutex_group_id', 26);
            $table->string('unit_id', 256);
            $table->char('claiming_experiment_id', 26);
            $table->timestamp('claimed_at')->useCurrent();
            $table->primary(['mutex_group_id', 'unit_id']);
            $table->foreign('mutex_group_id')->references('id')->on('mutex_groups')->cascadeOnDelete();
            $table->foreign('claiming_experiment_id')->references('id')->on('experiments')->cascadeOnDelete();
        });
    }

    public function down(): void
    {
        Schema::dropIfExists('mutex_group_assignments');
        DB::statement('DROP TABLE IF EXISTS assignments CASCADE;');
    }
};