// Package alias models application command-path rewriting independently of
// command execution and the parsing backend.
package alias

import (
	"fmt"

	"gitea.maximumdirect.net/eric/CommandKit/internal/commandpath"
)

// Mapping replaces Source with Target when Source is the longest matching
// prefix of an entered command path.
type Mapping struct {
	Source commandpath.Path
	Target commandpath.Path
}

type trieNode struct {
	children map[string]int
	mapping  int
	best     int
	parent   int
	path     commandpath.Path
}

// Resolver selects longest matching command-path mappings.
type Resolver struct {
	mappings []Mapping
	trie     []trieNode
}

// NewResolver builds a resolver from normalized command-path mappings.
func NewResolver(mappings []Mapping) (*Resolver, error) {
	resolver := &Resolver{trie: []trieNode{{mapping: -1, best: -1, parent: -1}}}
	for _, mapping := range mappings {
		if len(mapping.Source) == 0 {
			return nil, fmt.Errorf("command alias source path must not be empty")
		}
		if len(mapping.Target) == 0 {
			return nil, fmt.Errorf("command alias target path must not be empty")
		}
		for _, component := range append(mapping.Source.Clone(), mapping.Target...) {
			if err := commandpath.ValidateComponent(component); err != nil {
				return nil, err
			}
		}
		normalized := Mapping{Source: mapping.Source.Clone(), Target: mapping.Target.Clone()}
		index := len(resolver.mappings)
		resolver.mappings = append(resolver.mappings, normalized)
		nodeIndex := 0
		for _, component := range normalized.Source {
			node := &resolver.trie[nodeIndex]
			if node.children == nil {
				node.children = make(map[string]int)
			}
			childIndex, exists := node.children[component]
			if !exists {
				childIndex = len(resolver.trie)
				node.children[component] = childIndex
				childPath := append(resolver.trie[nodeIndex].path.Clone(), component)
				resolver.trie = append(resolver.trie, trieNode{mapping: -1, best: -1, parent: nodeIndex, path: childPath})
			}
			nodeIndex = childIndex
		}
		node := &resolver.trie[nodeIndex]
		if node.mapping >= 0 {
			previous := resolver.mappings[node.mapping]
			if previous.Target.Equal(normalized.Target) {
				resolver.mappings = resolver.mappings[:index]
				continue
			}
			return nil, fmt.Errorf("conflicting command aliases for %q", normalized.Source.String())
		}
		node.mapping = index
		node.path = normalized.Source.Clone()
	}
	resolver.setFallbacks()
	return resolver, nil
}

func (resolver *Resolver) setFallbacks() {
	for index := 0; index < len(resolver.trie); index++ {
		node := &resolver.trie[index]
		if index > 0 {
			node.best = resolver.trie[node.parent].best
		}
		if node.mapping >= 0 {
			node.best = node.mapping
		}
	}
}

// ResolveOnce applies at most one longest-prefix mapping. It returns the
// original input unchanged when no mapping matches.
func (resolver *Resolver) ResolveOnce(path commandpath.Path) (commandpath.Path, Mapping, bool) {
	if resolver == nil {
		return path.Clone(), Mapping{}, false
	}
	nodeIndex := 0
	matched := -1
	matchedLength := 0
	for index, component := range path {
		childIndex, exists := resolver.trie[nodeIndex].children[component]
		if !exists {
			break
		}
		nodeIndex = childIndex
		if mapping := resolver.trie[nodeIndex].mapping; mapping >= 0 {
			matched = mapping
			matchedLength = index + 1
		}
	}
	if matched < 0 {
		return path.Clone(), Mapping{}, false
	}
	mapping := resolver.mappings[matched]
	resolved := make(commandpath.Path, 0, len(mapping.Target)+len(path)-matchedLength)
	resolved = append(resolved, mapping.Target...)
	resolved = append(resolved, path[matchedLength:]...)
	return resolved, Mapping{Source: mapping.Source.Clone(), Target: mapping.Target.Clone()}, true
}

// Resolve applies mappings until no source matches. Installation validates
// termination before a resolver is used for application input.
func (resolver *Resolver) Resolve(path commandpath.Path) commandpath.Path {
	current := path.Clone()
	for {
		next, _, matched := resolver.ResolveOnce(current)
		if !matched {
			return current
		}
		current = next
	}
}

// ResolutionPaths returns the entered path and each path produced by a
// longest-prefix rewrite. It supports metadata discovery along alias chains.
// Installation validates termination before this method is used.
func (resolver *Resolver) ResolutionPaths(path commandpath.Path) []commandpath.Path {
	paths := []commandpath.Path{path.Clone()}
	for {
		next, _, matched := resolver.ResolveOnce(paths[len(paths)-1])
		if !matched {
			return paths
		}
		paths = append(paths, next)
	}
}

// ResolveWithProvenance resolves a command path and carries the input token
// indexes that contributed to each output component. Rewritten target
// components inherit the origins of the matched source prefix.
func (resolver *Resolver) ResolveWithProvenance(path commandpath.Path) (commandpath.Path, [][]int) {
	current := path.Clone()
	origins := make([][]int, len(path))
	for index := range path {
		origins[index] = []int{index}
	}
	for {
		next, mapping, matched := resolver.ResolveOnce(current)
		if !matched {
			return current, origins
		}

		matchedOrigins := make([]int, 0)
		seen := make(map[int]struct{})
		for _, componentOrigins := range origins[:len(mapping.Source)] {
			for _, origin := range componentOrigins {
				if _, exists := seen[origin]; exists {
					continue
				}
				seen[origin] = struct{}{}
				matchedOrigins = append(matchedOrigins, origin)
			}
		}
		nextOrigins := make([][]int, 0, len(mapping.Target)+len(origins)-len(mapping.Source))
		for range mapping.Target {
			nextOrigins = append(nextOrigins, append([]int(nil), matchedOrigins...))
		}
		nextOrigins = append(nextOrigins, origins[len(mapping.Source):]...)
		current = next
		origins = nextOrigins
	}
}