use candle_core::{Device, Tensor, DType};
use candle_nn::VarBuilder;
use candle_transformers::models::qwen2::{Config as QwenConfig, ModelForCausalLM as QwenModel};
use hf_hub::{api::sync::Api, Repo, RepoType};
use std::path::PathBuf;
use std::time::Instant;

pub struct LlmEngine {
    tokenizer: tokenizers::Tokenizer,
    model_path: PathBuf,
    device: Device,
    dtype: DType,
    config: QwenConfig,
    eos_token: u32,
}

impl LlmEngine {
    pub fn load() -> Result<Self, String> {
        let device = Device::cuda_if_available(0).map_err(|e| format!("Device: {}", e))?;
        let device_name = if device.is_cuda() { "CUDA" } else { "CPU" };
        tracing::info!("LLM device: {}", device_name);

        let dtype = if device.is_cuda() { DType::F16 } else { DType::F32 };

        tracing::info!("Ladataan Qwen2.5-0.5B-Instruct...");
        let api = Api::new().map_err(|e| format!("HF API: {}", e))?;
        let repo = api.repo(Repo::with_revision(
            "Qwen/Qwen2.5-0.5B-Instruct".to_string(),
            RepoType::Model,
            "main".to_string(),
        ));

        let tokenizer_path = repo.get("tokenizer.json").map_err(|e| format!("Tokenizer lataus: {}", e))?;
        let model_path = repo.get("model.safetensors").map_err(|e| format!("Malli lataus: {}", e))?;

        tracing::info!("Ladataan tokenizer: {:?}", tokenizer_path);
        let tokenizer = tokenizers::Tokenizer::from_file(&tokenizer_path)
            .map_err(|e| format!("Tokenizer: {}", e))?;

        let config = QwenConfig {
            vocab_size: 151936,
            hidden_size: 896,
            intermediate_size: 4864,
            num_hidden_layers: 24,
            num_attention_heads: 14,
            num_key_value_heads: 2,
            max_position_embeddings: 32768,
            sliding_window: 32768,
            max_window_layers: 21,
            tie_word_embeddings: true,
            rope_theta: 1000000.0,
            rms_norm_eps: 1e-6,
            use_sliding_window: false,
            hidden_act: candle_nn::Activation::Silu,
        };

        // Testi-lataus varmistaa, että painot toimivat
        let start = Instant::now();
        let vb = unsafe {
            VarBuilder::from_mmaped_safetensors(&[model_path.clone()], dtype, &device)
                .map_err(|e| format!("VarBuilder: {}", e))?
        };
        let _model = QwenModel::new(&config, vb).map_err(|e| format!("Malli: {}", e))?;
        tracing::info!("Malli ladattu ({:.1}s) — {}", start.elapsed().as_secs_f64(), device_name);

        Ok(LlmEngine {
            tokenizer,
            model_path,
            device,
            dtype,
            config,
            eos_token: 151645,
        })
    }

    /// Luo tuore malliinstanssi (nollaa KV-cachen)
    fn fresh_model(&self) -> Result<QwenModel, String> {
        let vb = unsafe {
            VarBuilder::from_mmaped_safetensors(&[self.model_path.clone()], self.dtype, &self.device)
                .map_err(|e| format!("VarBuilder: {}", e))?
        };
        QwenModel::new(&self.config, vb).map_err(|e| format!("Malli: {}", e))
    }

    pub fn generate(&mut self, prompt: &str, max_tokens: usize) -> Result<GenerateResult, String> {
        let formatted = format!("<|im_start|>user\n{}<|im_end|>\n<|im_start|>assistant\n", prompt);

        let encoding = self.tokenizer.encode(formatted.as_str(), true)
            .map_err(|e| format!("Encode: {}", e))?;
        let input_ids: Vec<u32> = encoding.get_ids().to_vec();
        let input_len = input_ids.len();

        // Tuore malli joka promptille (nollaa KV-cachen)
        let mut model = self.fresh_model()?;

        let start = Instant::now();

        // Prefill
        let input = Tensor::new(input_ids.as_slice(), &self.device)
            .and_then(|t| t.unsqueeze(0))
            .map_err(|e| format!("Tensor: {}", e))?;

        let logits = model.forward(&input, 0)
            .map_err(|e| format!("Forward prefill: {}", e))?;

        let logits = logits.squeeze(0).map_err(|e| format!("Squeeze: {}", e))?;
        let logits = if logits.dims().len() == 2 {
            logits.get(logits.dim(0).unwrap() - 1).map_err(|e| format!("Get: {}", e))?
        } else {
            logits
        };
        let mut next_token = logits.argmax(0)
            .map_err(|e| format!("Argmax: {}", e))?
            .to_vec0::<u32>()
            .map_err(|e| format!("to_vec0: {}", e))?;

        let mut generated_text = String::new();
        let mut tokens_generated: usize = 0;
        let mut all_tokens: Vec<u32> = Vec::new();

        if next_token != self.eos_token {
            if let Ok(text) = self.tokenizer.decode(&[next_token], true) {
                generated_text.push_str(&text);
            }
            all_tokens.push(next_token);
            tokens_generated += 1;
        }

        // Autoregressive
        let mut pos = input_len;
        for _ in 1..max_tokens {
            if next_token == self.eos_token { break; }

            let input = Tensor::new(&[next_token], &self.device)
                .and_then(|t| t.unsqueeze(0))
                .map_err(|e| format!("Tensor: {}", e))?;

            let logits = model.forward(&input, pos)
                .map_err(|e| format!("Forward pos {}: {}", pos, e))?;

            let logits = logits.squeeze(0).map_err(|e| format!("Squeeze: {}", e))?;
            let logits = if logits.dims().len() == 2 {
                logits.get(logits.dim(0).unwrap() - 1).map_err(|e| format!("Get: {}", e))?
            } else {
                logits
            };
            next_token = logits.argmax(0)
                .map_err(|e| format!("Argmax: {}", e))?
                .to_vec0::<u32>()
                .map_err(|e| format!("to_vec0: {}", e))?;
            pos += 1;

            if next_token == self.eos_token { break; }

            if let Ok(text) = self.tokenizer.decode(&[next_token], true) {
                generated_text.push_str(&text);
            }
            all_tokens.push(next_token);
            tokens_generated += 1;
        }

        let gen_time = start.elapsed();
        let tokens_per_sec = if gen_time.as_secs_f64() > 0.0 {
            tokens_generated as f64 / gen_time.as_secs_f64()
        } else { 0.0 };

        Ok(GenerateResult {
            text: generated_text,
            tokens_generated,
            duration_ms: gen_time.as_millis() as f64,
            tokens_per_sec,
        })
    }
}

pub struct GenerateResult {
    pub text: String,
    pub tokens_generated: usize,
    pub duration_ms: f64,
    pub tokens_per_sec: f64,
}