package agent

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"runtime"
	"strings"
	"sync"
	"time"

	"github.com/sipeed/picoclaw/pkg/logger"
	"github.com/sipeed/picoclaw/pkg/providers"
	"github.com/sipeed/picoclaw/pkg/skills"
)

type ContextBuilder struct {
	workspace    string
	skillsLoader *skills.SkillsLoader
	memory       *MemoryStore

	// Cache for system prompt to avoid rebuilding on every call.
	// This fixes issue #607: repeated reprocessing of the entire context.
	// The cache auto-invalidates when workspace source files change (mtime check).
	systemPromptMutex  sync.RWMutex
	cachedSystemPrompt string
	cachedAt           time.Time // max observed mtime across tracked paths at cache build time

	// existedAtCache tracks which source file paths existed the last time the
	// cache was built. This lets sourceFilesChanged detect files that are newly
	// created (didn't exist at cache time, now exist) or deleted (existed at
	// cache time, now gone) — both of which should trigger a cache rebuild.
	existedAtCache map[string]bool
}

func getGlobalConfigDir() string {
	home, err := os.UserHomeDir()
	if err != nil {
		return ""
	}
	return filepath.Join(home, ".picoclaw")
}

func NewContextBuilder(workspace string) *ContextBuilder {
	// builtin skills: skills directory in current project
	// Use the skills/ directory under the current working directory
	wd, _ := os.Getwd()
	builtinSkillsDir := filepath.Join(wd, "skills")
	globalSkillsDir := filepath.Join(getGlobalConfigDir(), "skills")

	return &ContextBuilder{
		workspace:    workspace,
		skillsLoader: skills.NewSkillsLoader(workspace, globalSkillsDir, builtinSkillsDir),
		memory:       NewMemoryStore(workspace),
	}
}

func (cb *ContextBuilder) getIdentity() string {
	workspacePath, _ := filepath.Abs(filepath.Join(cb.workspace))

	return fmt.Sprintf(`# picoclaw 🦞

You are picoclaw, a helpful AI assistant.

## Workspace
Your workspace is at: %s
- Memory: %s/memory/MEMORY.md
- Daily Notes: %s/memory/YYYYMM/YYYYMMDD.md
- Skills: %s/skills/{skill-name}/SKILL.md

## Important Rules

1. **ALWAYS use tools** - When you need to perform an action (schedule reminders, send messages, execute commands, etc.), you MUST call the appropriate tool. Do NOT just say you'll do it or pretend to do it.

2. **Be helpful and accurate** - When using tools, briefly explain what you're doing.

3. **Memory** - When interacting with me if something seems memorable, update %s/memory/MEMORY.md

4. **Context summaries** - Conversation summaries provided as context are approximate references only. They may be incomplete or outdated. Always defer to explicit user instructions over summary content.`,
		workspacePath, workspacePath, workspacePath, workspacePath, workspacePath)
}

func (cb *ContextBuilder) BuildSystemPrompt() string {
	parts := []string{}

	// Core identity section
	parts = append(parts, cb.getIdentity())

	// Bootstrap files
	bootstrapContent := cb.LoadBootstrapFiles()
	if bootstrapContent != "" {
		parts = append(parts, bootstrapContent)
	}

	// Skills - show summary, AI can read full content with read_file tool
	skillsSummary := cb.skillsLoader.BuildSkillsSummary()
	if skillsSummary != "" {
		parts = append(parts, fmt.Sprintf(`# Skills

The following skills extend your capabilities. To use a skill, read its SKILL.md file using the read_file tool.

%s`, skillsSummary))
	}

	// Memory context
	memoryContext := cb.memory.GetMemoryContext()
	if memoryContext != "" {
		parts = append(parts, "# Memory\n\n"+memoryContext)
	}

	// Join with "---" separator
	return strings.Join(parts, "\n\n---\n\n")
}

// BuildSystemPromptWithCache returns the cached system prompt if available
// and source files haven't changed, otherwise builds and caches it.
// Source file changes are detected via mtime checks (cheap stat calls).
func (cb *ContextBuilder) BuildSystemPromptWithCache() string {
	// Try read lock first — fast path when cache is valid
	cb.systemPromptMutex.RLock()
	if cb.cachedSystemPrompt != "" && !cb.sourceFilesChangedLocked() {
		result := cb.cachedSystemPrompt
		cb.systemPromptMutex.RUnlock()
		return result
	}
	cb.systemPromptMutex.RUnlock()

	// Acquire write lock for building
	cb.systemPromptMutex.Lock()
	defer cb.systemPromptMutex.Unlock()

	// Double-check: another goroutine may have rebuilt while we waited
	if cb.cachedSystemPrompt != "" && !cb.sourceFilesChangedLocked() {
		return cb.cachedSystemPrompt
	}

	// Snapshot the baseline (existence + max mtime) BEFORE building the prompt.
	// This way cachedAt reflects the pre-build state: if a file is modified
	// during BuildSystemPrompt, its new mtime will be > baseline.maxMtime,
	// so the next sourceFilesChangedLocked check will correctly trigger a
	// rebuild. The alternative (baseline after build) risks caching stale
	// content with a too-new baseline, making the staleness invisible.
	baseline := cb.buildCacheBaseline()
	prompt := cb.BuildSystemPrompt()
	cb.cachedSystemPrompt = prompt
	cb.cachedAt = baseline.maxMtime
	cb.existedAtCache = baseline.existed

	logger.DebugCF("agent", "System prompt cached",
		map[string]any{
			"length": len(prompt),
		})

	return prompt
}

// InvalidateCache clears the cached system prompt.
// Normally not needed because the cache auto-invalidates via mtime checks,
// but this is useful for tests or explicit reload commands.
func (cb *ContextBuilder) InvalidateCache() {
	cb.systemPromptMutex.Lock()
	defer cb.systemPromptMutex.Unlock()

	cb.cachedSystemPrompt = ""
	cb.cachedAt = time.Time{}
	cb.existedAtCache = nil

	logger.DebugCF("agent", "System prompt cache invalidated", nil)
}

// sourcePaths returns the workspace source file paths tracked for cache
// invalidation (bootstrap files + memory). The skills directory is handled
// separately in sourceFilesChangedLocked because it requires both directory-
// level and recursive file-level mtime checks.
func (cb *ContextBuilder) sourcePaths() []string {
	return []string{
		filepath.Join(cb.workspace, "AGENTS.md"),
		filepath.Join(cb.workspace, "SOUL.md"),
		filepath.Join(cb.workspace, "USER.md"),
		filepath.Join(cb.workspace, "IDENTITY.md"),
		filepath.Join(cb.workspace, "memory", "MEMORY.md"),
	}
}

// cacheBaseline holds the file existence snapshot and the latest observed
// mtime across all tracked paths. Used as the cache reference point.
type cacheBaseline struct {
	existed  map[string]bool
	maxMtime time.Time
}

// buildCacheBaseline records which tracked paths currently exist and computes
// the latest mtime across all tracked files + skills directory contents.
// Called under write lock when the cache is built.
func (cb *ContextBuilder) buildCacheBaseline() cacheBaseline {
	skillsDir := filepath.Join(cb.workspace, "skills")

	// All paths whose existence we track: source files + skills dir.
	allPaths := append(cb.sourcePaths(), skillsDir)

	existed := make(map[string]bool, len(allPaths))
	var maxMtime time.Time

	for _, p := range allPaths {
		info, err := os.Stat(p)
		existed[p] = err == nil
		if err == nil && info.ModTime().After(maxMtime) {
			maxMtime = info.ModTime()
		}
	}

	// Walk skills files to capture their mtimes too.
	// Use os.Stat (not d.Info) to match the stat method used in
	// fileChangedSince / skillFilesModifiedSince for consistency.
	_ = filepath.WalkDir(skillsDir, func(path string, d fs.DirEntry, walkErr error) error {
		if walkErr == nil && !d.IsDir() {
			if info, err := os.Stat(path); err == nil && info.ModTime().After(maxMtime) {
				maxMtime = info.ModTime()
			}
		}
		return nil
	})

	// If no tracked files exist yet (empty workspace), maxMtime is zero.
	// Use a very old non-zero time so that:
	// 1. cachedAt.IsZero() won't trigger perpetual rebuilds.
	// 2. Any real file created afterwards has mtime > cachedAt, so it
	//    will be detected by fileChangedSince (unlike time.Now() which
	//    could race with a file whose mtime <= Now).
	if maxMtime.IsZero() {
		maxMtime = time.Unix(1, 0)
	}

	return cacheBaseline{existed: existed, maxMtime: maxMtime}
}

// sourceFilesChangedLocked checks whether any workspace source file has been
// modified, created, or deleted since the cache was last built.
//
// IMPORTANT: The caller MUST hold at least a read lock on systemPromptMutex.
// Go's sync.RWMutex is not reentrant, so this function must NOT acquire the
// lock itself (it would deadlock when called from BuildSystemPromptWithCache
// which already holds RLock or Lock).
func (cb *ContextBuilder) sourceFilesChangedLocked() bool {
	if cb.cachedAt.IsZero() {
		return true
	}

	// Check tracked source files (bootstrap + memory).
	for _, p := range cb.sourcePaths() {
		if cb.fileChangedSince(p) {
			return true
		}
	}

	// --- Skills directory (handled separately from sourcePaths) ---
	//
	// 1. Creation/deletion: tracked via existedAtCache, same as bootstrap files.
	skillsDir := filepath.Join(cb.workspace, "skills")
	if cb.fileChangedSince(skillsDir) {
		return true
	}

	// 2. Structural changes (add/remove entries inside the dir) are reflected
	//    in the directory's own mtime, which fileChangedSince already checks.
	//
	// 3. Content-only edits to files inside skills/ do NOT update the parent
	//    directory mtime on most filesystems, so we recursively walk to check
	//    individual file mtimes at any nesting depth.
	if skillFilesModifiedSince(skillsDir, cb.cachedAt) {
		return true
	}

	return false
}

// fileChangedSince returns true if a tracked source file has been modified,
// newly created, or deleted since the cache was built.
//
// Four cases:
//   - existed at cache time, exists now -> check mtime
//   - existed at cache time, gone now   -> changed (deleted)
//   - absent at cache time,  exists now -> changed (created)
//   - absent at cache time,  gone now   -> no change
func (cb *ContextBuilder) fileChangedSince(path string) bool {
	// Defensive: if existedAtCache was never initialized, treat as changed
	// so the cache rebuilds rather than silently serving stale data.
	if cb.existedAtCache == nil {
		return true
	}

	existedBefore := cb.existedAtCache[path]
	info, err := os.Stat(path)
	existsNow := err == nil

	if existedBefore != existsNow {
		return true // file was created or deleted
	}
	if !existsNow {
		return false // didn't exist before, doesn't exist now
	}
	return info.ModTime().After(cb.cachedAt)
}

// errWalkStop is a sentinel error used to stop filepath.WalkDir early.
// Using a dedicated error (instead of fs.SkipAll) makes the early-exit
// intent explicit and avoids the nilerr linter warning that would fire
// if the callback returned nil when its err parameter is non-nil.
var errWalkStop = errors.New("walk stop")

// skillFilesModifiedSince recursively walks the skills directory and checks
// whether any file was modified after t. This catches content-only edits at
// any nesting depth (e.g. skills/name/docs/extra.md) that don't update
// parent directory mtimes.
func skillFilesModifiedSince(skillsDir string, t time.Time) bool {
	changed := false
	err := filepath.WalkDir(skillsDir, func(path string, d fs.DirEntry, walkErr error) error {
		if walkErr == nil && !d.IsDir() {
			if info, statErr := os.Stat(path); statErr == nil && info.ModTime().After(t) {
				changed = true
				return errWalkStop // stop walking
			}
		}
		return nil
	})
	// errWalkStop is expected (early exit on first changed file).
	// os.IsNotExist means the skills dir doesn't exist yet — not an error.
	// Any other error is unexpected and worth logging.
	if err != nil && !errors.Is(err, errWalkStop) && !os.IsNotExist(err) {
		logger.DebugCF("agent", "skills walk error", map[string]any{"error": err.Error()})
	}
	return changed
}

func (cb *ContextBuilder) LoadBootstrapFiles() string {
	bootstrapFiles := []string{
		"AGENTS.md",
		"SOUL.md",
		"USER.md",
		"IDENTITY.md",
	}

	var sb strings.Builder
	for _, filename := range bootstrapFiles {
		filePath := filepath.Join(cb.workspace, filename)
		if data, err := os.ReadFile(filePath); err == nil {
			fmt.Fprintf(&sb, "## %s\n\n%s\n\n", filename, data)
		}
	}

	return sb.String()
}

// buildDynamicContext returns a short dynamic context string with per-request info.
// This changes every request (time, session) so it is NOT part of the cached prompt.
// LLM-side KV cache reuse is achieved by each provider adapter's native mechanism:
//   - Anthropic: per-block cache_control (ephemeral) on the static SystemParts block
//   - OpenAI / Codex: prompt_cache_key for prefix-based caching
//
// See: https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching
// See: https://platform.openai.com/docs/guides/prompt-caching
func (cb *ContextBuilder) buildDynamicContext(channel, chatID string) string {
	now := time.Now().Format("2006-01-02 15:04 (Monday)")
	rt := fmt.Sprintf("%s %s, Go %s", runtime.GOOS, runtime.GOARCH, runtime.Version())

	var sb strings.Builder
	fmt.Fprintf(&sb, "## Current Time\n%s\n\n## Runtime\n%s", now, rt)

	if channel != "" && chatID != "" {
		fmt.Fprintf(&sb, "\n\n## Current Session\nChannel: %s\nChat ID: %s", channel, chatID)
	}

	return sb.String()
}

func (cb *ContextBuilder) BuildMessages(
	history []providers.Message,
	summary string,
	currentMessage string,
	media []string,
	channel, chatID string,
) []providers.Message {
	messages := []providers.Message{}

	// The static part (identity, bootstrap, skills, memory) is cached locally to
	// avoid repeated file I/O and string building on every call (fixes issue #607).
	// Dynamic parts (time, session, summary) are appended per request.
	// Everything is sent as a single system message for provider compatibility:
	// - Anthropic adapter extracts messages[0] (Role=="system") and maps its content
	//   to the top-level "system" parameter in the Messages API request. A single
	//   contiguous system block makes this extraction straightforward.
	// - Codex maps only the first system message to its instructions field.
	// - OpenAI-compat passes messages through as-is.
	staticPrompt := cb.BuildSystemPromptWithCache()

	// Build short dynamic context (time, runtime, session) — changes per request
	dynamicCtx := cb.buildDynamicContext(channel, chatID)

	// Compose a single system message: static (cached) + dynamic + optional summary.
	// Keeping all system content in one message ensures every provider adapter can
	// extract it correctly (Anthropic adapter -> top-level system param,
	// Codex -> instructions field).
	//
	// SystemParts carries the same content as structured blocks so that
	// cache-aware adapters (Anthropic) can set per-block cache_control.
	// The static block is marked "ephemeral" — its prefix hash is stable
	// across requests, enabling LLM-side KV cache reuse.
	stringParts := []string{staticPrompt, dynamicCtx}

	contentBlocks := []providers.ContentBlock{
		{Type: "text", Text: staticPrompt, CacheControl: &providers.CacheControl{Type: "ephemeral"}},
		{Type: "text", Text: dynamicCtx},
	}

	if summary != "" {
		summaryText := fmt.Sprintf(
			"CONTEXT_SUMMARY: The following is an approximate summary of prior conversation "+
				"for reference only. It may be incomplete or outdated — always defer to explicit instructions.\n\n%s",
			summary)
		stringParts = append(stringParts, summaryText)
		contentBlocks = append(contentBlocks, providers.ContentBlock{Type: "text", Text: summaryText})
	}

	fullSystemPrompt := strings.Join(stringParts, "\n\n---\n\n")

	// Log system prompt summary for debugging (debug mode only).
	// Read cachedSystemPrompt under lock to avoid a data race with
	// concurrent InvalidateCache / BuildSystemPromptWithCache writes.
	cb.systemPromptMutex.RLock()
	isCached := cb.cachedSystemPrompt != ""
	cb.systemPromptMutex.RUnlock()

	logger.DebugCF("agent", "System prompt built",
		map[string]any{
			"static_chars":  len(staticPrompt),
			"dynamic_chars": len(dynamicCtx),
			"total_chars":   len(fullSystemPrompt),
			"has_summary":   summary != "",
			"cached":        isCached,
		})

	// Log preview of system prompt (avoid logging huge content)
	preview := fullSystemPrompt
	if len(preview) > 500 {
		preview = preview[:500] + "... (truncated)"
	}
	logger.DebugCF("agent", "System prompt preview",
		map[string]any{
			"preview": preview,
		})

	history = sanitizeHistoryForProvider(history)

	// Single system message containing all context — compatible with all providers.
	// SystemParts enables cache-aware adapters to set per-block cache_control;
	// Content is the concatenated fallback for adapters that don't read SystemParts.
	messages = append(messages, providers.Message{
		Role:        "system",
		Content:     fullSystemPrompt,
		SystemParts: contentBlocks,
	})

	// Add conversation history
	messages = append(messages, history...)

	// Add current user message
	if strings.TrimSpace(currentMessage) != "" {
		messages = append(messages, providers.Message{
			Role:    "user",
			Content: currentMessage,
		})
	}

	return messages
}

func sanitizeHistoryForProvider(history []providers.Message) []providers.Message {
	if len(history) == 0 {
		return history
	}

	sanitized := make([]providers.Message, 0, len(history))
	for _, msg := range history {
		switch msg.Role {
		case "system":
			// Drop system messages from history. BuildMessages always
			// constructs its own single system message (static + dynamic +
			// summary); extra system messages would break providers that
			// only accept one (Anthropic, Codex).
			logger.DebugCF("agent", "Dropping system message from history", map[string]any{})
			continue

		case "tool":
			if len(sanitized) == 0 {
				logger.DebugCF("agent", "Dropping orphaned leading tool message", map[string]any{})
				continue
			}
			// Walk backwards to find the nearest assistant message,
			// skipping over any preceding tool messages (multi-tool-call case).
			foundAssistant := false
			for i := len(sanitized) - 1; i >= 0; i-- {
				if sanitized[i].Role == "tool" {
					continue
				}
				if sanitized[i].Role == "assistant" && len(sanitized[i].ToolCalls) > 0 {
					foundAssistant = true
				}
				break
			}
			if !foundAssistant {
				logger.DebugCF("agent", "Dropping orphaned tool message", map[string]any{})
				continue
			}
			sanitized = append(sanitized, msg)

		case "assistant":
			if len(msg.ToolCalls) > 0 {
				if len(sanitized) == 0 {
					logger.DebugCF("agent", "Dropping assistant tool-call turn at history start", map[string]any{})
					continue
				}
				prev := sanitized[len(sanitized)-1]
				if prev.Role != "user" && prev.Role != "tool" {
					logger.DebugCF(
						"agent",
						"Dropping assistant tool-call turn with invalid predecessor",
						map[string]any{"prev_role": prev.Role},
					)
					continue
				}
			}
			sanitized = append(sanitized, msg)

		default:
			sanitized = append(sanitized, msg)
		}
	}

	return sanitized
}

func (cb *ContextBuilder) AddToolResult(
	messages []providers.Message,
	toolCallID, toolName, result string,
) []providers.Message {
	messages = append(messages, providers.Message{
		Role:       "tool",
		Content:    result,
		ToolCallID: toolCallID,
	})
	return messages
}

func (cb *ContextBuilder) AddAssistantMessage(
	messages []providers.Message,
	content string,
	toolCalls []map[string]any,
) []providers.Message {
	msg := providers.Message{
		Role:    "assistant",
		Content: content,
	}
	// Always add assistant message, whether or not it has tool calls
	messages = append(messages, msg)
	return messages
}

// GetSkillsInfo returns information about loaded skills.
func (cb *ContextBuilder) GetSkillsInfo() map[string]any {
	allSkills := cb.skillsLoader.ListSkills()
	skillNames := make([]string, 0, len(allSkills))
	for _, s := range allSkills {
		skillNames = append(skillNames, s.Name)
	}
	return map[string]any{
		"total":     len(allSkills),
		"available": len(allSkills),
		"names":     skillNames,
	}
}