unit std.intern;

import std.map;
import std.mem;

// One copy of every distinct name, and a number that stands for it.
//
// A compiler compares names constantly and stores them everywhere. Comparing
// two `StrId` is comparing two integers; storing one costs four bytes and no
// ownership. That is the whole point.
//
// There is deliberately no way to get a `str` back out. A borrow of the text
// would be a borrow of the interner, and the interner is exactly the thing you
// want to keep adding to while holding names -- the parser reads an identifier
// and registers the next one in the same breath. Everything you would open the
// text for is here instead: compare it, measure it, hash it, write it.

/// A name, as a number. Copy, four bytes, and meaningless to any other
/// interner -- which is fine, because a program has one.
pub struct StrId {
    pub raw: u32,

    pub fn same(self: &Self, other: StrId) -> bool {
        return self.raw == other.raw;
    }
}

/// No name.
pub const NONE: u32 = 4294967295;

struct Entry {
    at: usize,
    len: usize,
}

pub struct Interner {
    /// Every name end to end. Nothing is ever removed, so an offset stays
    /// good for as long as the interner does.
    bytes: ^[]mut u8,
    used: usize,
    names: ^[]mut Entry,
    count: usize,
    /// Text to id, so interning the same name twice gives the same number.
    seen: map.Map(u32),

    pub fn with_capacity(n: usize) -> !Self {
        let text: ^[]mut u8 = try mem.alloc_slice(u8, n * 8);
        let table: ^[]mut Entry = try mem.alloc_slice(Entry, n);
        let index: map.Map(u32) = try map.Map(u32).with_capacity(n);
        return Self{ bytes: text, used: 0, names: table, count: 0,
                     seen: index };
    }

    pub fn count_of(self: &Self) -> usize { return self.count; }

    /// The number for this name, making one if it is new.
    pub fn intern(self: &mut Self, text: []u8) -> !StrId {
        let found: u32 = self.seen.get(text, NONE);
        if found != NONE { return StrId{ raw: found }; }
        if self.count == self.names.^.n { return error.OutOfMemory; }
        let at: usize = self.used;
        if at + text.n > self.bytes.^.n { return error.OutOfMemory; }
        var i: usize = 0;
        while i < text.n {
            self.bytes.^[at + i] = text[i];
            i = i + 1;
        }
        let id: usize = self.count;
        self.names.^[id].at = at;
        self.names.^[id].len = text.n;
        self.used = at + text.n;
        self.count = id + 1;
        try self.seen.put(text, id as u32);
        return StrId{ raw: id as u32 };
    }

    /// Is this a name it has seen? `NONE` when not.
    pub fn find(self: &Self, text: []u8) -> u32 {
        return self.seen.get(text, NONE);
    }

    pub fn len_of(self: &Self, id: StrId) -> usize {
        if (id.raw as usize) >= self.count { return 0; }
        return self.names.^[id.raw as usize].len;
    }

    /// Does this id spell this text? The comparison every `if name == "fn"`
    /// in a parser turns into.
    pub fn eq(self: &Self, id: StrId, text: []u8) -> bool {
        if (id.raw as usize) >= self.count { return false; }
        let e: usize = id.raw as usize;
        if self.names.^[e].len != text.n { return false; }
        var i: usize = 0;
        while i < text.n {
            if self.bytes.^[self.names.^[e].at + i] != text[i] { return false; }
            i = i + 1;
        }
        return true;
    }

    /// FNV-1a over the stored bytes, for anything that wants to bucket names
    /// without opening them.
    pub fn hash_of(self: &Self, id: StrId) -> u32 {
        if (id.raw as usize) >= self.count { return 0; }
        let e: usize = id.raw as usize;
        var h: u32 = 2166136261;
        var i: usize = 0;
        while i < self.names.^[e].len {
            h = h ^ (self.bytes.^[self.names.^[e].at + i] as u32);
            h = h * 16777619;
            i = i + 1;
        }
        return h;
    }

    /// Copy the name into `out` and say how many bytes it took. This is how a
    /// name reaches a diagnostic without the interner being borrowed past the
    /// statement.
    pub fn copy_into(self: &Self, id: StrId, out: []mut u8) -> usize {
        if (id.raw as usize) >= self.count { return 0; }
        let e: usize = id.raw as usize;
        var n: usize = self.names.^[e].len;
        if n > out.n { n = out.n; }
        var i: usize = 0;
        while i < n {
            out[i] = self.bytes.^[self.names.^[e].at + i];
            i = i + 1;
        }
        return n;
    }
}