package agents

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"os"

	yaml "gopkg.in/yaml.v3"
)

// MergeYAML is the YAML cousin of MergeJSON / MergeTOML, with one
// important difference: it preserves comments and structure. Hermes
// config (~/.hermes/config.yaml) is comment-rich and hand-edited, so
// a round-trip through map[string]any — which silently drops every
// comment and reorders keys — would mangle the user's file. Instead
// we decode into a *yaml.Node tree (which carries HeadComment /
// LineComment / FootComment on every node) and re-encode it, so an
// idempotent re-run is a no-op and a real merge touches only the keys
// we add.
//
// mutate receives the top-level mapping node (created empty for a new
// or empty file) and reports whether it changed anything. The agents
// package ships node helpers — YAMLMapValue / YAMLSetMapValue /
// YAMLScalar / UpsertYAMLMapEntry — so callers don't hand-walk the
// Content slice.
//
// Malformed YAML is preserved as a .bak sibling before we overwrite,
// same policy as MergeJSON / MergeTOML. A valid-but-non-mapping
// top-level document (a bare scalar or sequence — never a real config
// file) is also backed up before we replace it with a fresh mapping.
func MergeYAML(w io.Writer, path string, mutate func(root *yaml.Node, existed bool) (changed bool, err error), opts ApplyOpts) (FileAction, error) {
	existed := false
	var backupPath string
	var backupData []byte
	indent := defaultYAMLIndent
	var doc yaml.Node

	if data, err := os.ReadFile(path); err == nil {
		existed = true
		if len(bytes.TrimSpace(data)) > 0 {
			indent = detectYAMLIndent(data)
			if err := yaml.Unmarshal(data, &doc); err != nil {
				// Malformed YAML — capture it for a .bak (written only in
				// the real-apply branch below, never under --dry-run) and
				// start from an empty document.
				backupPath, backupData = path+".bak", data
				doc = yaml.Node{}
			} else if !documentHasMappingRoot(&doc) {
				// Valid YAML but not a top-level mapping (a bare list
				// or scalar). We can't safely splice mcp_servers into
				// that, so preserve it and start fresh.
				backupPath, backupData = path+".bak", data
				doc = yaml.Node{}
			}
		}
	} else if !errors.Is(err, os.ErrNotExist) {
		return FileAction{}, fmt.Errorf("read %s: %w", path, err)
	}

	root := documentRoot(&doc)

	changed, err := mutate(root, existed)
	if err != nil {
		return FileAction{}, err
	}
	if !changed {
		return FileAction{Path: path, Action: ActionSkip, Reason: "already-configured"}, nil
	}

	keys := yamlTopLevelKeys(root)

	if opts.DryRun {
		action := ActionWouldCreate
		if existed {
			action = ActionWouldMerge
		}
		return FileAction{Path: path, Action: action, Keys: keys}, nil
	}

	out, err := marshalYAMLDocument(&doc, indent)
	if err != nil {
		return FileAction{}, fmt.Errorf("marshal %s: %w", path, err)
	}
	// Preserve the unparseable / non-mapping original as a .bak sibling
	// before we overwrite it. Deferred to here, the real-apply branch,
	// so a --dry-run on a malformed file never mutates disk.
	if backupPath != "" {
		if err := os.WriteFile(backupPath, backupData, 0o644); err != nil {
			return FileAction{}, fmt.Errorf("write backup %s: %w", backupPath, err)
		}
	}
	if err := AtomicWriteFile(path, out, 0o644); err != nil {
		return FileAction{}, err
	}

	action := ActionCreate
	if existed {
		action = ActionMerge
	}
	if backupPath != "" {
		logf(w, "[gortex init] %s was malformed YAML; backup saved to %s", path, backupPath)
	}
	logf(w, "[gortex init] %s %s", actionVerb(action), path)
	return FileAction{Path: path, Action: action, Keys: keys}, nil
}

// documentHasMappingRoot reports whether a freshly-unmarshaled
// document node carries a mapping at its root — the only shape we can
// safely merge into.
func documentHasMappingRoot(doc *yaml.Node) bool {
	return doc.Kind == yaml.DocumentNode &&
		len(doc.Content) > 0 &&
		doc.Content[0].Kind == yaml.MappingNode
}

// documentRoot normalises doc so it is a DocumentNode wrapping a
// MappingNode and returns that mapping. A zero-value node (empty /
// absent file) is turned into an empty document so the caller always
// gets a writable mapping back.
func documentRoot(doc *yaml.Node) *yaml.Node {
	if documentHasMappingRoot(doc) {
		return doc.Content[0]
	}
	mapping := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
	doc.Kind = yaml.DocumentNode
	doc.Content = []*yaml.Node{mapping}
	return mapping
}

// marshalYAMLDocument renders a document node back to bytes at the given
// indent width — matched to the file's own (see detectYAMLIndent) so a
// merge re-emits 4-space configs as 4-space rather than forcing 2 —
// preserving the comments captured during Unmarshal.
func marshalYAMLDocument(doc *yaml.Node, indent int) ([]byte, error) {
	var buf bytes.Buffer
	enc := yaml.NewEncoder(&buf)
	enc.SetIndent(indent)
	if err := enc.Encode(doc); err != nil {
		_ = enc.Close()
		return nil, err
	}
	if err := enc.Close(); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// defaultYAMLIndent is the indentation width MergeYAML falls back to
// when a file's own indent can't be inferred (a new or single-level
// file). 2 is what Hermes and most hand-written YAML use.
const defaultYAMLIndent = 2

// detectYAMLIndent infers the indentation step (in spaces) a YAML file
// already uses, so a comment-preserving merge re-emits the file at the
// width its author chose. It returns the smallest positive leading-space
// count across non-comment lines — for well-formed YAML that is the
// per-level step — clamped to a sane range, else defaultYAMLIndent.
//
// This matches the indent *width* only. yaml.v3's encoder cannot emit
// indentless block sequences (a key whose `- item` children sit at the
// key's own column); those are always re-indented one level on a merge.
// That is a known go-yaml limitation, not something this can undo — a
// fully diff-free merge would require a surgical byte-splice instead of
// a whole-document re-encode.
func detectYAMLIndent(data []byte) int {
	best := 0
	for line := range bytes.SplitSeq(data, []byte("\n")) {
		n := 0
		for n < len(line) && line[n] == ' ' {
			n++
		}
		if n == 0 || n >= len(line) { // top-level, blank, or all spaces
			continue
		}
		if line[n] == '#' { // comments may be aligned arbitrarily
			continue
		}
		if best == 0 || n < best {
			best = n
		}
	}
	if best < 1 || best > 8 {
		return defaultYAMLIndent
	}
	return best
}

// yamlTopLevelKeys returns the key names of a mapping node, in file
// order. Used to populate FileAction.Keys for the --json report.
func yamlTopLevelKeys(m *yaml.Node) []string {
	if m == nil || m.Kind != yaml.MappingNode {
		return nil
	}
	out := make([]string, 0, len(m.Content)/2)
	for i := 0; i+1 < len(m.Content); i += 2 {
		out = append(out, m.Content[i].Value)
	}
	return out
}

// YAMLMapValue returns the value node stored under key in a mapping
// node, or nil when the key is absent (or m isn't a mapping). YAML
// mappings are stored as a flat [k0, v0, k1, v1, …] Content slice;
// this hides that layout from callers.
func YAMLMapValue(m *yaml.Node, key string) *yaml.Node {
	if m == nil || m.Kind != yaml.MappingNode {
		return nil
	}
	for i := 0; i+1 < len(m.Content); i += 2 {
		if m.Content[i].Value == key {
			return m.Content[i+1]
		}
	}
	return nil
}

// YAMLSetMapValue sets key=val in a mapping node, replacing an
// existing value in place (so its leading comment survives) or
// appending a new key/value pair.
func YAMLSetMapValue(m *yaml.Node, key string, val *yaml.Node) {
	for i := 0; i+1 < len(m.Content); i += 2 {
		if m.Content[i].Value == key {
			m.Content[i+1] = val
			return
		}
	}
	m.Content = append(m.Content, YAMLScalar(key), val)
}

// YAMLScalar builds a plain string scalar node. Callers that need
// non-string scalars (ints, bools) construct yaml.Node literals with
// the matching tag directly.
func YAMLScalar(value string) *yaml.Node {
	return &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: value}
}

// UpsertYAMLMapEntry ensures root[outerKey][innerKey] = entry, where
// outerKey's value is a nested mapping (created when absent). Returns
// true when the tree was modified, false when innerKey already exists
// and force is off — the idempotent-re-run signal MergeYAML turns
// into an "already-configured" skip.
//
// This is the YAML analogue of UpsertMCPServer: Hermes stores MCP
// servers under the snake_case `mcp_servers` map rather than the
// camelCase `mcpServers` every JSON client uses.
func UpsertYAMLMapEntry(root *yaml.Node, outerKey, innerKey string, entry *yaml.Node, force bool) (changed bool, err error) {
	outer := YAMLMapValue(root, outerKey)
	switch {
	case outer == nil:
		// Key absent — create the nested mapping.
		outer = &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
		YAMLSetMapValue(root, outerKey, outer)
	case outer.Kind == yaml.MappingNode:
		// Reuse the existing mapping in place.
	case isNullYAMLNode(outer):
		// `outerKey:` with an empty / null value — e.g. every server
		// commented out so the map decodes to null. Safe to populate:
		// replacing the null value in place keeps the key node (and its
		// leading comment) intact.
		outer = &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
		YAMLSetMapValue(root, outerKey, outer)
	default:
		// A non-null scalar or a sequence under outerKey is not a shape
		// we can splice a child into. Refuse rather than silently drop
		// the user's data — the caller leaves the file untouched.
		return false, fmt.Errorf("agents: %q is %s, not a mapping; refusing to overwrite", outerKey, yamlKindName(outer.Kind))
	}
	if existing := YAMLMapValue(outer, innerKey); existing != nil && !force {
		return false, nil
	}
	YAMLSetMapValue(outer, innerKey, entry)
	return true, nil
}

// isNullYAMLNode reports whether n is a YAML null — an explicitly null
// scalar (`~`, `null`) or an empty value (`key:` with nothing after it).
func isNullYAMLNode(n *yaml.Node) bool {
	return n != nil && n.Kind == yaml.ScalarNode && (n.Tag == "!!null" || n.Value == "")
}

// yamlKindName renders a node kind for a human-readable error message.
func yamlKindName(k yaml.Kind) string {
	switch k {
	case yaml.SequenceNode:
		return "a sequence"
	case yaml.ScalarNode:
		return "a scalar"
	case yaml.MappingNode:
		return "a mapping"
	case yaml.AliasNode:
		return "an alias"
	default:
		return "a non-mapping value"
	}
}