const debug = @import("debug.zig");
const assert = debug.assert;
const math = @import("math.zig");
const mem = @import("mem.zig");
const Allocator = mem.Allocator;

const want_modification_safety = !@compileVar("is_release");
const debug_u32 = if (want_modification_safety) u32 else void;

pub fn HashMap(comptime K: type, comptime V: type,
    comptime hash: fn(key: K)->u32,
    comptime eql: fn(a: K, b: K)->bool) -> type
{
    struct {
        entries: []Entry,
        size: usize,
        max_distance_from_start_index: usize,
        allocator: &Allocator,
        // this is used to detect bugs where a hashtable is edited while an iterator is running.
        modification_count: debug_u32,

        const Self = this;

        pub const Entry = struct {
            used: bool,
            distance_from_start_index: usize,
            key: K,
            value: V,
        };

        pub const Iterator = struct {
            hm: &Self,
            // how many items have we returned
            count: usize,
            // iterator through the entry array
            index: usize,
            // used to detect concurrent modification
            initial_modification_count: debug_u32,

            pub fn next(it: &Iterator) -> ?&Entry {
                if (want_modification_safety) {
                    assert(it.initial_modification_count == it.hm.modification_count); // concurrent modification
                }
                if (it.count >= it.hm.size) return null;
                while (it.index < it.hm.entries.len; it.index += 1) {
                    const entry = &it.hm.entries[it.index];
                    if (entry.used) {
                        it.index += 1;
                        it.count += 1;
                        return entry;
                    }
                }
                @unreachable() // no next item
            }
        };

        pub fn init(hm: &Self, allocator: &Allocator) {
            hm.entries = []Entry{};
            hm.allocator = allocator;
            hm.size = 0;
            hm.max_distance_from_start_index = 0;
            // it doesn't actually matter what we set this to since we use wrapping integer arithmetic
            hm.modification_count = undefined;
        }

        pub fn deinit(hm: &Self) {
            hm.allocator.free(hm.entries);
        }

        pub fn clear(hm: &Self) {
            for (hm.entries) |*entry| {
                entry.used = false;
            }
            hm.size = 0;
            hm.max_distance_from_start_index = 0;
            hm.incrementModificationCount();
        }

        pub fn put(hm: &Self, key: K, value: V) -> %void {
            if (hm.entries.len == 0) {
                %return hm.initCapacity(16);
            }
            hm.incrementModificationCount();

            // if we get too full (60%), double the capacity
            if (hm.size * 5 >= hm.entries.len * 3) {
                const old_entries = hm.entries;
                %return hm.initCapacity(hm.entries.len * 2);
                // dump all of the old elements into the new table
                for (old_entries) |*old_entry| {
                    if (old_entry.used) {
                        hm.internalPut(old_entry.key, old_entry.value);
                    }
                }
                hm.allocator.free(old_entries);
            }

            hm.internalPut(key, value);
        }

        pub fn get(hm: &Self, key: K) -> ?&Entry {
            return hm.internalGet(key);
        }

        pub fn remove(hm: &Self, key: K) {
            hm.incrementModificationCount();
            const start_index = hm.keyToIndex(key);
            {var roll_over: usize = 0; while (roll_over <= hm.max_distance_from_start_index; roll_over += 1) {
                const index = (start_index + roll_over) % hm.entries.len;
                var entry = &hm.entries[index];

                assert(entry.used); // key not found

                if (!eql(entry.key, key)) continue;

                while (roll_over < hm.entries.len; roll_over += 1) {
                    const next_index = (start_index + roll_over + 1) % hm.entries.len;
                    const next_entry = &hm.entries[next_index];
                    if (!next_entry.used || next_entry.distance_from_start_index == 0) {
                        entry.used = false;
                        hm.size -= 1;
                        return;
                    }
                    *entry = *next_entry;
                    entry.distance_from_start_index -= 1;
                    entry = next_entry;
                }
                @unreachable() // shifting everything in the table
            }}
            @unreachable() // key not found
        }

        pub fn entryIterator(hm: &Self) -> Iterator {
            return Iterator {
                .hm = hm,
                .count = 0,
                .index = 0,
                .initial_modification_count = hm.modification_count,
            };
        }

        fn initCapacity(hm: &Self, capacity: usize) -> %void {
            hm.entries = %return hm.allocator.alloc(Entry, capacity);
            hm.size = 0;
            hm.max_distance_from_start_index = 0;
            for (hm.entries) |*entry| {
                entry.used = false;
            }
        }

        fn incrementModificationCount(hm: &Self) {
            if (want_modification_safety) {
                hm.modification_count +%= 1;
            }
        }

        fn internalPut(hm: &Self, orig_key: K, orig_value: V) {
            var key = orig_key;
            var value = orig_value;
            const start_index = hm.keyToIndex(key);
            var roll_over: usize = 0;
            var distance_from_start_index: usize = 0;
            while (roll_over < hm.entries.len; {roll_over += 1; distance_from_start_index += 1}) {
                const index = (start_index + roll_over) % hm.entries.len;
                const entry = &hm.entries[index];

                if (entry.used && !eql(entry.key, key)) {
                    if (entry.distance_from_start_index < distance_from_start_index) {
                        // robin hood to the rescue
                        const tmp = *entry;
                        hm.max_distance_from_start_index = math.max(hm.max_distance_from_start_index,
                            distance_from_start_index);
                        *entry = Entry {
                            .used = true,
                            .distance_from_start_index = distance_from_start_index,
                            .key = key,
                            .value = value,
                        };
                        key = tmp.key;
                        value = tmp.value;
                        distance_from_start_index = tmp.distance_from_start_index;
                    }
                    continue;
                }

                if (!entry.used) {
                    // adding an entry. otherwise overwriting old value with
                    // same key
                    hm.size += 1;
                }

                hm.max_distance_from_start_index = math.max(distance_from_start_index, hm.max_distance_from_start_index);
                *entry = Entry {
                    .used = true,
                    .distance_from_start_index = distance_from_start_index,
                    .key = key,
                    .value = value,
                };
                return;
            }
            @unreachable() // put into a full map
        }

        fn internalGet(hm: &Self, key: K) -> ?&Entry {
            const start_index = hm.keyToIndex(key);
            {var roll_over: usize = 0; while (roll_over <= hm.max_distance_from_start_index; roll_over += 1) {
                const index = (start_index + roll_over) % hm.entries.len;
                const entry = &hm.entries[index];

                if (!entry.used) return null;
                if (eql(entry.key, key)) return entry;
            }}
            return null;
        }

        fn keyToIndex(hm: &Self, key: K) -> usize {
            return usize(hash(key)) % hm.entries.len;
        }
    }
}

test "basicHashMapTest" {
    var map: HashMap(i32, i32, hash_i32, eql_i32) = undefined;
    map.init(&debug.global_allocator);
    defer map.deinit();

    %%map.put(1, 11);
    %%map.put(2, 22);
    %%map.put(3, 33);
    %%map.put(4, 44);
    %%map.put(5, 55);

    assert((??map.get(2)).value == 22);
    map.remove(2);
    assert(if (const entry ?= map.get(2)) false else true);
}

fn hash_i32(x: i32) -> u32 {
    *(&u32)(&x)
}
fn eql_i32(a: i32, b: i32) -> bool {
    a == b
}