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;

// MaxShells is the maximum number of "popular" shells.
const MaxShells = 63;
const MaxShellLen = 64;

// ShellIndex is an index to the shell strings. As shell can be up to 64 bytes
// (1<<6), maximum number of shells is 63 (1<<6-1), the maximum location offset
// is 1<<12. To make location resolvable in 10 bits, all shells will be padded
// to 4 bytes.
// The actual shell length is len+1: we don't allow empty shells, and the real
// length of the shell is 1-64 bytes.
const ShellIndex = struct {
    offset: u10,
    len: u6,
};

// ShellReader interprets "Shell Index" and "Shell Blob" sections.
const ShellReader = struct {
    sectionIndex: []const ShellIndex,
    sectionBlob: []const u8,

    pub fn init(index: []const u8, blob: []const u8) ShellReader {
        return ShellReader{
            .sectionIndex = @bitCast([]const ShellIndex, index),
            .sectionBlob = blob,
        };
    }

    // get returns a shell at the given index.
    pub fn get(self: *const ShellReader, idx: u10) []const u8 {
        const shellIndex = self.sectionIndex[idx];
        const start = shellIndex.offset << 2;
        const end = start + shellIndex.len + 1;
        return self.sectionBlob[start..end];
    }
};

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

    const ShellSections = struct {
        index: BoundedArray(ShellIndex, MaxShells),
        blob: BoundedArray(u8, MaxShells * MaxShellLen),
        indices: StringHashMap(u10),

        // initializes and populates shell sections. All strings are copied,
        // nothing is owned.
        pub fn init(
            allocator: Allocator,
            shells: BoundedArray([]const u8, MaxShells),
        ) !ShellSections {
            var self = ShellSections{
                .index = try BoundedArray(ShellIndex, MaxShells).init(shells.len),
                .blob = try BoundedArray(u8, MaxShells * MaxShellLen).init(0),
                .indices = StringHashMap(u10).init(allocator),
            };
            var fullOffset: u12 = 0;
            var idx: u10 = 0;
            while (idx < shells.len) {
                const len = @intCast(u6, shells.get(idx).len);
                try self.blob.appendSlice(shells.get(idx));
                const ourShell = self.blob.constSlice()[fullOffset .. fullOffset + len];
                try self.indices.put(ourShell, idx);
                self.index.set(idx, ShellIndex{
                    .offset = @intCast(u10, fullOffset >> 2),
                    .len = len - 1,
                });

                fullOffset += len;
                const padding = roundUp4Padding(fullOffset);
                fullOffset += padding;
                //const stderr = std.io.getStdErr().writer();
                //try stderr.print("\n", .{});
                try self.blob.appendNTimes(0, padding);
                idx += 1;
            }
            return self;
        }

        pub fn sectionIndex(self: *const ShellSections) []const u8 {
            return @bitCast([]const u8, self.index.constSlice());
        }

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

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

        pub fn getIndex(self: *const ShellSections, shell: []const u8) ?u10 {
            return self.indices.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.value_ptr.* += 1;
        } else {
            // TODO(motiejus): can we avoid `ourShell` variable here?
            const ourShell = try self.allocator.alloc(u8, shell.len);
            std.mem.copy(u8, ourShell, shell);
            res.key_ptr.* = ourShell;
            res.value_ptr.* = 1;
        }
    }

    fn cmpShells(context: void, a: KV, b: KV) std.math.Order {
        _ = context;
        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) !ShellSections {
        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 = @truncate(u32, 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 = try BoundedArray([]const u8, MaxShells).init(total);

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

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

// rounds up a u12 to the nearest factor of 4 and returns the difference
// (padding)
inline fn roundUp4Padding(n: u12) u12 {
    return ((n + 3) & ~@intCast(u12, 3)) - n;
}

const testing = std.testing;

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

    const bash = "/bin/bash"; // 9 chars
    const zsh = "/bin/zsh"; // 8 chars
    const nobody = "/bin/nobody"; // only 1 instance, ought to ignore
    const long = "/bin/very-long-shell-name-ought-to-be-first";
    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*42=84
    };

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

    var sections = try popcon.toOwnedSections(MaxShells);
    defer sections.deinit();
    try testing.expectEqual(sections.index.len, 3); // 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);

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

test "padding" {
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 0)), 0);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 1)), 3);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 2)), 2);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 3)), 1);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 4)), 0);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 40)), 0);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 41)), 3);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 42)), 2);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 43)), 1);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 44)), 0);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 4091)), 1);
    try testing.expectEqual(roundUp4Padding(@intCast(u12, 4092)), 0);
}