#=
model.jl — Self-contained inference engine for JuliaSLM

Implements the full Lux-trained JuliaGPT architecture (RoPE, RMSNorm, SwiGLU,
weight-tied decoder-only transformer) using only NNlib primitives.
No Lux dependency required — parameters are loaded directly from JLD2.
CPU-only inference.

Supports both character-level and BPE tokenizers.
=#

using NNlib
using NNlib: batched_mul
using Statistics
using Random
using JSON3
using TOML

# ═══════════════════════════════════════════════════════════════════
# Model configuration
# ═══════════════════════════════════════════════════════════════════

struct ModelConfig
    embed_dim::Int
    n_layers::Int
    n_heads::Int
    head_dim::Int
    context_length::Int
    vocab_size::Int
    weight_tying::Bool
    bias::Bool
end

function load_config_toml(path::String; vocab_size::Int=0)
    raw = TOML.parsefile(path)
    m = get(raw, "model", Dict())
    return ModelConfig(
        get(m, "embed_dim", 256),
        get(m, "n_layers", 6),
        get(m, "n_heads", 4),
        get(m, "head_dim", 64),
        get(m, "context_length", 256),
        vocab_size,
        get(m, "weight_tying", true),
        get(m, "bias", false),
    )
end

# ═══════════════════════════════════════════════════════════════════
# Character-level tokenizer
# ═══════════════════════════════════════════════════════════════════

struct CharTokenizer
    char_to_idx::Dict{Char, Int}
    idx_to_char::Vector{Char}
    vocab_size::Int
end

function load_char_vocab_json(path::String)
    raw = JSON3.read(read(path, String))
    chars = Char[only(String(s)) for s in raw]
    char_to_idx = Dict(c => i for (i, c) in enumerate(chars))
    return CharTokenizer(char_to_idx, chars, length(chars))
end

function encode(t::CharTokenizer, text::String)
    indices = Int[]
    sizehint!(indices, length(text))
    for c in text
        idx = get(t.char_to_idx, c, nothing)
        idx !== nothing && push!(indices, idx)
    end
    return indices
end

function decode(t::CharTokenizer, indices::AbstractVector{<:Integer})
    buf = IOBuffer()
    for idx in indices
        if 1 <= idx <= t.vocab_size
            write(buf, t.idx_to_char[idx])
        end
    end
    return String(take!(buf))
end

# ═══════════════════════════════════════════════════════════════════
# BPE Tokenizer (GPT-2 style)
# ═══════════════════════════════════════════════════════════════════

struct BPETokenizer
    encoder::Dict{String, Int}     # token string → id (0-indexed from file)
    decoder::Dict{Int, String}     # id → token string (0-indexed)
    merges::Vector{Tuple{String, String}}
    merge_ranks::Dict{Tuple{String, String}, Int}
    byte_to_unicode::Dict{UInt8, Char}
    unicode_to_byte::Dict{Char, UInt8}
    vocab_size::Int
    pat::Regex
end

function load_bpe_tokenizer(vocab_path::String, merges_path::String)
    encoder = JSON3.read(read(vocab_path, String), Dict{String, Int})
    decoder = Dict{Int, String}(v => k for (k, v) in encoder)

    merge_lines = readlines(merges_path)
    start = startswith(first(merge_lines), "#") ? 2 : 1
    merges = Tuple{String, String}[]
    for line in merge_lines[start:end]
        parts = split(strip(line))
        length(parts) == 2 && push!(merges, (String(parts[1]), String(parts[2])))
    end
    merge_ranks = Dict{Tuple{String, String}, Int}(m => i for (i, m) in enumerate(merges))

    b2u = _build_byte_to_unicode()
    u2b = Dict{Char, UInt8}(v => k for (k, v) in b2u)

    pat = r"'s|'t|'re|'ve|'m|'ll|'d| ?\p{L}+| ?\p{N}+| ?[^\s\p{L}\p{N}]+|\s+(?!\S)|\s+"

    return BPETokenizer(encoder, decoder, merges, merge_ranks, b2u, u2b,
                        length(encoder), pat)
end

function encode(t::BPETokenizer, text::String)
    tokens = Int[]
    for m in eachmatch(t.pat, text)
        word = m.match
        encoded_chars = [string(t.byte_to_unicode[b]) for b in Vector{UInt8}(word)]
        bpe_tokens = _bpe_encode_word(encoded_chars, t.merge_ranks)
        for tok in bpe_tokens
            id = get(t.encoder, tok, nothing)
            id !== nothing && push!(tokens, id + 1)  # +1 for Julia 1-indexing
        end
    end
    return tokens
end

function decode(t::BPETokenizer, ids::AbstractVector{<:Integer})
    token_strs = [get(t.decoder, id - 1, "") for id in ids]  # -1 to undo Julia 1-indexing
    joined = join(token_strs)
    out = UInt8[]
    sizehint!(out, length(joined))
    for c in joined
        b = get(t.unicode_to_byte, c, nothing)
        if b !== nothing
            push!(out, b)
        else
            append!(out, codeunits(string(c)))
        end
    end
    return String(out)
end

function _bpe_encode_word(symbols::Vector{String}, merge_ranks::Dict{Tuple{String, String}, Int})
    while length(symbols) > 1
        best_pair = nothing
        best_rank = typemax(Int)
        for i in 1:length(symbols)-1
            pair = (symbols[i], symbols[i+1])
            rank = get(merge_ranks, pair, typemax(Int))
            if rank < best_rank
                best_rank = rank
                best_pair = pair
            end
        end
        best_rank == typemax(Int) && break

        new_symbols = String[]
        i = 1
        while i <= length(symbols)
            if i < length(symbols) && symbols[i] == best_pair[1] && symbols[i+1] == best_pair[2]
                push!(new_symbols, best_pair[1] * best_pair[2])
                i += 2
            else
                push!(new_symbols, symbols[i])
                i += 1
            end
        end
        symbols = new_symbols
    end
    return symbols
end

function _build_byte_to_unicode()
    bs = UInt8[]
    append!(bs, UInt8('!'):UInt8('~'))
    append!(bs, UInt8('¡'):UInt8('¬'))
    append!(bs, UInt8('®'):UInt8('ÿ'))
    cs = Int[Int(b) for b in bs]
    n = 0
    for b in 0x00:0xff
        if !(b in bs)
            push!(bs, b)
            push!(cs, 256 + n)
            n += 1
        end
    end
    return Dict{UInt8, Char}(b => Char(c) for (b, c) in zip(bs, cs))
end

# ═══════════════════════════════════════════════════════════════════
# Unified tokenizer interface
# ═══════════════════════════════════════════════════════════════════

const Tokenizer = Union{CharTokenizer, BPETokenizer}

tokenizer_vocab_size(t::CharTokenizer) = t.vocab_size
tokenizer_vocab_size(t::BPETokenizer) = t.vocab_size

# ═══════════════════════════════════════════════════════════════════
# Causal mask
# ═══════════════════════════════════════════════════════════════════

function make_causal_mask(seq_len::Int)
    return Float32[j <= i ? 0.0f0 : typemin(Float32) for i in 1:seq_len, j in 1:seq_len]
end

# ═══════════════════════════════════════════════════════════════════
# Rotary Positional Embedding
# ═══════════════════════════════════════════════════════════════════

function compute_rope_caches(head_dim::Int, max_seq_len::Int)
    freqs = Float32.(1.0 ./ (10000.0 .^ ((0:2:(head_dim-1)) ./ head_dim)))
    positions = Float32.(0:(max_seq_len-1))
    angles = freqs * positions'
    return cos.(angles), sin.(angles)
end

function apply_rotary_emb(x, cos_cache, sin_cache, seq_len)
    half = size(x, 1) ÷ 2
    x1 = x[1:half, :, :, :]
    x2 = x[half+1:end, :, :, :]
    c = cos_cache[:, 1:seq_len]
    s = sin_cache[:, 1:seq_len]
    o1 = x1 .* c .- x2 .* s
    o2 = x1 .* s .+ x2 .* c
    return vcat(o1, o2)
end

# ═══════════════════════════════════════════════════════════════════
# Layer primitives
# ═══════════════════════════════════════════════════════════════════

function rmsnorm_forward(x, weight; eps=1.0f-6)
    rms = sqrt.(mean(x .^ 2; dims=1) .+ eps)
    return weight .* (x ./ rms)
end

function swiglu_forward(x, ps)
    D = size(x, 1)
    x_flat = reshape(x, D, :)
    gate = ps.w1 * x_flat
    val  = ps.v  * x_flat
    hidden = NNlib.swish.(gate) .* val
    out = ps.w2 * hidden
    return reshape(out, D, size(x)[2:end]...)
end

function self_attention_forward(x, ps, n_heads, head_dim, rope_cos, rope_sin, mask)
    D, T, B = size(x)
    H = n_heads
    HD = head_dim

    x_flat = reshape(x, D, T * B)
    q = reshape(ps.wq * x_flat, HD, T, H, B)
    k = reshape(ps.wk * x_flat, HD, T, H, B)
    v = reshape(ps.wv * x_flat, HD, T, H, B)

    q = apply_rotary_emb(q, rope_cos, rope_sin, T)
    k = apply_rotary_emb(k, rope_cos, rope_sin, T)

    scale = Float32(1.0 / sqrt(Float64(HD)))
    q_r = reshape(q, HD, T, H * B)
    k_r = reshape(k, HD, T, H * B)
    attn = batched_mul(permutedims(q_r, (2, 1, 3)), k_r) .* scale
    attn = attn .+ mask
    attn = NNlib.softmax(attn; dims=2)

    v_r = reshape(v, HD, T, H * B)
    out = batched_mul(v_r, permutedims(attn, (2, 1, 3)))
    out = reshape(out, HD * H, T, B)
    result = reshape(ps.wo * reshape(out, HD * H, T * B), D, T, B)
    return result
end

# ═══════════════════════════════════════════════════════════════════
# Full model forward pass (uses pre-computed causal mask)
# ═══════════════════════════════════════════════════════════════════

function model_forward(config::ModelConfig, ps, rope_cos, rope_sin, x, causal_mask)
    T = size(x, 1)  # x: (seq_len, batch) of integer token IDs

    # Token embedding: (seq_len, batch) → (embed_dim, seq_len, batch)
    h = ps.tok_emb.weight[:, x]

    # Slice pre-computed causal mask to actual sequence length
    mask = causal_mask[1:T, 1:T]

    # Transformer blocks
    for i in 1:config.n_layers
        name = Symbol("block_$i")
        bp = getproperty(ps.blocks, name)

        # Pre-norm attention + residual
        normed = rmsnorm_forward(h, bp.ln1.weight)
        attn_out = self_attention_forward(normed, bp.attn,
                                          config.n_heads, config.head_dim,
                                          rope_cos, rope_sin, mask)
        h = h .+ attn_out

        # Pre-norm FFN + residual
        normed2 = rmsnorm_forward(h, bp.ln2.weight)
        ffn_out = swiglu_forward(normed2, bp.ffn)
        h = h .+ ffn_out
    end

    # Final norm
    h = rmsnorm_forward(h, ps.ln_f.weight)

    # Output projection
    D, T_out, B = size(h)
    h_flat = reshape(h, D, T_out * B)
    if config.weight_tying
        logits = ps.tok_emb.weight' * h_flat
    else
        logits = ps.head.weight * h_flat
    end
    return reshape(logits, :, T_out, B)
end

# ═══════════════════════════════════════════════════════════════════
# Sampling helpers
# ═══════════════════════════════════════════════════════════════════

function top_k_filter(logits::AbstractVector, k::Int)
    k = min(k, length(logits))
    sorted = sort(Array(logits); rev=true)
    threshold = sorted[k]
    return map(l -> l >= threshold ? l : typemin(eltype(logits)), logits)
end

function top_p_filter(logits::AbstractVector, p::Float64)
    sorted_indices = sortperm(Array(logits); rev=true)
    sorted_logits = logits[sorted_indices]
    probs = NNlib.softmax(sorted_logits)
    cumprobs = cumsum(Array(probs))
    cutoff = something(findfirst(>=(p), cumprobs), length(probs))
    result = fill(typemin(eltype(logits)), length(logits))
    for i in 1:cutoff
        result[sorted_indices[i]] = logits[sorted_indices[i]]
    end
    return result
end

function sample_categorical(probs::AbstractVector)
    r = rand(Float32)
    cumulative = 0.0f0
    for i in eachindex(probs)
        cumulative += probs[i]
        r <= cumulative && return i
    end
    return length(probs)
end

# ═══════════════════════════════════════════════════════════════════
# Text generation with streaming callback
# ═══════════════════════════════════════════════════════════════════

function generate_streaming(config::ModelConfig, ps, rope_cos, rope_sin,
                            tokenizer::Tokenizer, prompt::String;
                            max_tokens::Int=200,
                            temperature::Float64=0.8,
                            top_k::Int=0,
                            top_p::Float64=1.0,
                            on_token=nothing,
                            causal_mask=nothing)
    tokens = encode(tokenizer, prompt)
    if isempty(tokens)
        tokens = [rand(1:tokenizer_vocab_size(tokenizer))]
    end

    # Use provided mask or compute once
    if causal_mask === nothing
        causal_mask = make_causal_mask(config.context_length)
    end

    generated = String[]

    for _ in 1:max_tokens
        ctx = if length(tokens) > config.context_length
            tokens[end-config.context_length+1:end]
        else
            copy(tokens)
        end

        x = reshape(ctx, :, 1)
        logits = model_forward(config, ps, rope_cos, rope_sin, x, causal_mask)
        next_logits = Vector{Float32}(logits[:, end, 1])

        # Temperature scaling
        if temperature != 1.0
            next_logits ./= Float32(temperature)
        end

        # Top-k filtering
        if top_k > 0
            next_logits = top_k_filter(next_logits, top_k)
        end

        # Top-p (nucleus) filtering
        if top_p < 1.0
            next_logits = top_p_filter(next_logits, top_p)
        end

        probs = NNlib.softmax(next_logits)
        next_token = sample_categorical(probs)

        push!(tokens, next_token)
        token_str = decode(tokenizer, [next_token])
        push!(generated, token_str)

        on_token !== nothing && on_token(token_str)
    end

    return join(generated)
end