const std = @import("std");
const Allocator = std.mem.Allocator;
const PriorityDequeue = std.PriorityDequeue;
const StringArrayHashMap = std.StringArrayHashMap;
const StringHashMap = std.StringHashMap;
const BoundedArray = std.BoundedArray;
const StringContext = std.hash_map.StringContext;
const assert = std.debug.assert;

pub const max_shells = 255;
pub const max_shell_len = 256;

// ShellReader interprets "Shell Index" and "Shell Blob" sections.
pub const ShellReader = struct {
    index: []const u16,
    blob: []const u8,

    pub fn init(index: []align(2) const u8, blob: []const u8) ShellReader {
        return ShellReader{
            .index = std.mem.bytesAsSlice(u16, index),
            .blob = blob,
        };
    }

    // get returns a shell at the given index.
    pub fn get(self: *const ShellReader, idx: u8) []const u8 {
        return self.blob[self.index[idx]..self.index[idx + 1]];
    }
};

// ShellWriter is a shell popularity contest: collect shells and return the
// popular ones, sorted by score. score := len(shell) * number_of_shells.
pub const ShellWriter = struct {
    counts: std.StringHashMap(u32),
    allocator: Allocator,
    const KV = struct {
        shell: []const u8,
        score: u64,
    };

    pub const ShellSections = struct {
        // len is the number of shells in this section.
        len: u8,
        // index points the i'th shell to it's offset in blob. The last
        // byte of the i'th shell is index[i+1].
        index: BoundedArray(u16, max_shells),
        // blob contains `index.len+1` number of records. The last record is
        // pointing to the end of the blob, so length of the last shell can be
        // calculated from the index array.
        blob: BoundedArray(u8, (max_shells + 1) * max_shell_len),
        // shell2idx helps translate a shell (string) to it's index.
        shell2idx: StringHashMap(u8),

        // initializes and populates shell sections. All strings are copied,
        // nothing is owned.
        pub fn init(
            allocator: Allocator,
            shells: BoundedArray([]const u8, max_shells),
        ) error{OutOfMemory}!ShellSections {
            assert(shells.len <= max_shells);
            var self = ShellSections{
                .len = @intCast(u8, shells.len),
                .index = BoundedArray(u16, max_shells).init(shells.len) catch unreachable,
                .blob = BoundedArray(u8, (max_shells + 1) * max_shell_len).init(0) catch unreachable,
                .shell2idx = StringHashMap(u8).init(allocator),
            };
            if (shells.len == 0) return self;

            errdefer self.shell2idx.deinit();
            for (shells.constSlice()) |shell, idx| {
                const idx8 = @intCast(u8, idx);
                const offset = @intCast(u16, self.blob.len);
                self.blob.appendSliceAssumeCapacity(shell);
                try self.shell2idx.put(self.blob.constSlice()[offset..], idx8);
                self.index.set(idx8, offset);
            }
            self.index.appendAssumeCapacity(@intCast(u8, self.blob.len));
            return self;
        }

        pub fn section_index(self: *const ShellSections) []align(2) const u8 {
            return std.mem.sliceAsBytes(self.index.constSlice());
        }

        pub fn section_blob(self: *const ShellSections) []const u8 {
            return self.blob.constSlice();
        }

        pub fn deinit(self: *ShellSections) void {
            self.shell2idx.deinit();
            self.* = undefined;
        }

        pub fn getIndex(self: *const ShellSections, shell: []const u8) ?u8 {
            return self.shell2idx.get(shell);
        }
    };

    pub fn init(allocator: Allocator) ShellWriter {
        return ShellWriter{
            .counts = std.StringHashMap(u32).init(allocator),
            .allocator = allocator,
        };
    }

    pub fn deinit(self: *ShellWriter) void {
        var it = self.counts.keyIterator();
        while (it.next()) |key_ptr|
            self.counts.allocator.free(key_ptr.*);
        self.counts.deinit();
        self.* = undefined;
    }

    pub fn put(self: *ShellWriter, shell: []const u8) !void {
        const res = try self.counts.getOrPutAdapted(shell, self.counts.ctx);
        if (!res.found_existing) {
            res.key_ptr.* = try self.allocator.dupe(u8, shell);
            res.value_ptr.* = 1;
        } else {
            res.value_ptr.* += 1;
        }
    }

    fn cmpShells(_: void, a: KV, b: KV) std.math.Order {
        return std.math.order(a.score, b.score);
    }

    // toOwnedSections returns the analyzed ShellSections. Resets the shell
    // popularity contest. ShellSections memory is allocated by the ShellWriter
    // allocator, and must be deInit'ed by the caller.
    pub fn toOwnedSections(self: *ShellWriter, limit: u10) error{OutOfMemory}!ShellSections {
        assert(limit <= max_shells);

        var deque = PriorityDequeue(KV, void, cmpShells).init(self.allocator, {});
        defer deque.deinit();

        var it = self.counts.iterator();
        while (it.next()) |entry| {
            if (entry.value_ptr.* == 1)
                continue;
            const score = entry.key_ptr.*.len * entry.value_ptr.*;
            try deque.add(KV{ .shell = entry.key_ptr.*, .score = score });
        }

        const total = std.math.min(deque.count(), limit);
        var topShells = BoundedArray([]const u8, max_shells).init(total) catch |err| switch (err) {
            error.Overflow => unreachable,
        };

        var i: u32 = 0;
        while (i < total) : (i += 1)
            topShells.set(i, deque.removeMax().shell);

        const result = ShellSections.init(self.allocator, topShells);
        self.deinit();
        self.* = init(self.allocator);
        return result;
    }
};

const testing = std.testing;

test "basic shellpopcon" {
    var popcon = ShellWriter.init(testing.allocator);

    const bash = "/bin/bash"; // 9 chars
    const zsh = "/bin/zsh"; // 8 chars
    const long = "/bin/very-long-shell-name-ought-to-be-first"; // 43 chars
    const nobody = "/bin/nobody"; // only 1 instance, ought to ignore
    const input = [_][]const u8{
        zsh, zsh, zsh, zsh, // zsh score 8*4=32
        bash, bash, bash, nobody, // bash score 3*9=27
        long, long, // long score 2*43=86
    };

    for (input) |shell| {
        try popcon.put(shell);
    }

    var sections = try popcon.toOwnedSections(max_shells);
    defer sections.deinit();
    try testing.expectEqual(sections.index.len, 4); // all but "nobody" qualify

    try testing.expectEqual(sections.getIndex(long).?, 0);
    try testing.expectEqual(sections.getIndex(zsh).?, 1);
    try testing.expectEqual(sections.getIndex(bash).?, 2);
    try testing.expectEqual(sections.getIndex(nobody), null);
    try testing.expectEqual(sections.section_blob().len, bash.len + zsh.len + long.len);

    const shellReader = ShellReader.init(
        sections.section_index(),
        sections.section_blob(),
    );
    try testing.expectEqualStrings(shellReader.get(0), long);
    try testing.expectEqualStrings(shellReader.get(1), zsh);
    try testing.expectEqualStrings(shellReader.get(2), bash);

    try testing.expectEqual(shellReader.index.len, 4);
}