package engine

import (
	"bytes"
	"encoding/binary"
	"errors"
	"math/big"
	"sort"
	"strings"

	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
)

var (
	errEmptySequenceIdentityFields      = errors.New("sequence identity requires at least one field")
	errInvalidSequenceIdentityNumber    = errors.New("invalid sequence identity number")
	errUnsupportedSequenceIdentityValue = errors.New("unsupported sequence identity value")
)

type sequenceIdentityDiagnosticCodes struct {
	invalid  Code
	conflict Code
}

type sequenceIdentityRecord struct {
	tuple    string
	index    int
	location document.Location
}

// normalizeSequenceIdentityFields treats identity field names as a set.
func normalizeSequenceIdentityFields(fields []string) ([]string, error) {
	if len(fields) == 0 {
		return nil, errEmptySequenceIdentityFields
	}
	normalized := append([]string(nil), fields...)
	sort.Strings(normalized)
	write := 0
	for _, field := range normalized {
		if write != 0 && normalized[write-1] == field {
			continue
		}
		normalized[write] = field
		write++
	}
	return normalized[:write], nil
}

// normalizeSequenceIdentityNumber returns coefficient and exponent without
// expanding powers of ten. The represented value is coefficient * 10^exponent.
func normalizeSequenceIdentityNumber(value string) (string, error) {
	if value == "" {
		return "", errInvalidSequenceIdentityNumber
	}
	negative := false
	if value[0] == '-' {
		negative = true
		value = value[1:]
	}
	if value == "" {
		return "", errInvalidSequenceIdentityNumber
	}

	exponent := new(big.Int)
	if index := strings.IndexAny(value, "eE"); index >= 0 {
		if strings.IndexAny(value[index+1:], "eE") >= 0 {
			return "", errInvalidSequenceIdentityNumber
		}
		text := value[index+1:]
		if text == "" {
			return "", errInvalidSequenceIdentityNumber
		}
		if _, ok := exponent.SetString(text, 10); !ok {
			return "", errInvalidSequenceIdentityNumber
		}
		value = value[:index]
	}

	point := strings.IndexByte(value, '.')
	if point >= 0 && strings.IndexByte(value[point+1:], '.') >= 0 {
		return "", errInvalidSequenceIdentityNumber
	}
	integerPart := value
	fractionPart := ""
	if point >= 0 {
		integerPart = value[:point]
		fractionPart = value[point+1:]
	}
	if integerPart == "" || (point >= 0 && fractionPart == "") {
		return "", errInvalidSequenceIdentityNumber
	}
	if len(integerPart) > 1 && integerPart[0] == '0' {
		return "", errInvalidSequenceIdentityNumber
	}
	digits := integerPart + fractionPart
	for _, digit := range digits {
		if digit < '0' || digit > '9' {
			return "", errInvalidSequenceIdentityNumber
		}
	}
	coefficient := strings.TrimLeft(digits, "0")
	if coefficient == "" {
		return "0e0", nil
	}
	exponent.Sub(exponent, big.NewInt(int64(len(fractionPart))))
	trimmed := strings.TrimRight(coefficient, "0")
	exponent.Add(exponent, big.NewInt(int64(len(coefficient)-len(trimmed))))
	coefficient = trimmed
	if negative {
		coefficient = "-" + coefficient
	}
	return coefficient + "e" + exponent.String(), nil
}

func sequenceScalarIdentityComponent(node *document.Node) (byte, string, error) {
	if node == nil {
		return 0, "", errUnsupportedSequenceIdentityValue
	}
	switch node.Kind {
	case document.String:
		return 's', node.Text, nil
	case document.Number:
		normalized, err := normalizeSequenceIdentityNumber(node.Text)
		return 'n', normalized, err
	case document.Boolean:
		if node.Text != "true" && node.Text != "false" {
			return 0, "", errUnsupportedSequenceIdentityValue
		}
		return 'b', node.Text, nil
	case document.Null:
		return 'z', "", nil
	default:
		return 0, "", errUnsupportedSequenceIdentityValue
	}
}

func sequenceIdentityTuple(values []*document.Node) (string, error) {
	var encoded bytes.Buffer
	for _, value := range values {
		kind, component, err := sequenceScalarIdentityComponent(value)
		if err != nil {
			return "", err
		}
		encoded.WriteByte(kind)
		var length [binary.MaxVarintLen64]byte
		n := binary.PutUvarint(length[:], uint64(len(component)))
		encoded.Write(length[:n])
		encoded.WriteString(component)
	}
	return encoded.String(), nil
}

// sequenceMappingIndex is built once per record. Structural fields are omitted
// by entry index, so a directive is excluded only at its actual occurrence.
type sequenceMappingIndex struct {
	entries []document.Entry
	byKey   map[string]int
}

func newSequenceMappingIndex(record *document.Node) sequenceMappingIndex {
	index := sequenceMappingIndex{byKey: make(map[string]int)}
	if record == nil || record.Kind != document.Mapping {
		return index
	}
	index.entries = record.Mapping
	directiveIndex := -1
	if directive, exists := inheritanceDirective(record); exists {
		for entryIndex := range record.Mapping {
			if record.Mapping[entryIndex].Key == directive.Key && record.Mapping[entryIndex].KeyLocation == directive.KeyLocation {
				directiveIndex = entryIndex
				break
			}
		}
	}
	for entryIndex, entry := range record.Mapping {
		if entryIndex == directiveIndex {
			continue
		}
		if _, exists := index.byKey[entry.Key]; !exists {
			index.byKey[entry.Key] = entryIndex
		}
	}
	return index
}

func (index sequenceMappingIndex) identity(fields []string) (string, []document.Location, string, int) {
	values := make([]*document.Node, len(fields))
	locations := make([]document.Location, len(fields))
	for fieldIndex, field := range fields {
		entryIndex, exists := index.byKey[field]
		if !exists {
			return "", nil, field, -1
		}
		entry := index.entries[entryIndex]
		if entry.Value == nil || (entry.Value.Kind != document.String && entry.Value.Kind != document.Number && entry.Value.Kind != document.Boolean && entry.Value.Kind != document.Null) {
			return "", nil, field, entryIndex
		}
		if _, _, err := sequenceScalarIdentityComponent(entry.Value); err != nil {
			return "", nil, field, entryIndex
		}
		values[fieldIndex] = entry.Value
		locations[fieldIndex] = entry.ValueLocation
		if entry.Value != nil && entry.Value.Location != (document.Location{}) {
			locations[fieldIndex] = entry.Value.Location
		}
	}
	tuple, err := sequenceIdentityTuple(values)
	if err != nil {
		return "", nil, fields[0], -2
	}
	return tuple, locations, "", -3
}

// validateSequenceIdentity checks mapping shape, direct identity fields, and
// uniqueness in one tuple-to-first-location index. It does not alter the nodes.
func validateSequenceIdentity(sequence *document.Node, path Path, fields []string, codes sequenceIdentityDiagnosticCodes) (map[string]sequenceIdentityRecord, []Diagnostic) {
	return validateSequenceIdentityFrom(sequence, path, fields, codes, 0)
}

func validateSequenceIdentityFrom(sequence *document.Node, path Path, fields []string, codes sequenceIdentityDiagnosticCodes, startIndex int) (map[string]sequenceIdentityRecord, []Diagnostic) {
	normalized, err := normalizeSequenceIdentityFields(fields)
	if err != nil {
		location := (*Location)(nil)
		if sequence != nil {
			converted := publicLocation(sequence.Location)
			location = &converted
		}
		return nil, []Diagnostic{{Code: codes.invalid, Severity: SeverityError, Path: path, Message: "sequence identity requires at least one field", Primary: location}}
	}
	return validateNormalizedSequenceIdentityFrom(sequence, path, normalized, codes, startIndex)
}

func validateNormalizedSequenceIdentityFrom(sequence *document.Node, path Path, normalized []string, codes sequenceIdentityDiagnosticCodes, startIndex int) (map[string]sequenceIdentityRecord, []Diagnostic) {
	if sequence == nil || sequence.Kind != document.Sequence {
		location := (*Location)(nil)
		if sequence != nil {
			converted := publicLocation(sequence.Location)
			location = &converted
		}
		return nil, []Diagnostic{{Code: codes.invalid, Severity: SeverityError, Path: path, Message: "identified collection must be a sequence", Primary: location}}
	}

	firstByTuple := make(map[string]sequenceIdentityRecord, len(sequence.Sequence))
	for itemIndex, item := range sequence.Sequence {
		itemPath := path.Index(startIndex + itemIndex)
		if item == nil || item.Kind != document.Mapping {
			var primary *Location
			if item != nil {
				converted := publicLocation(item.Location)
				primary = &converted
			}
			return nil, []Diagnostic{{Code: codes.invalid, Severity: SeverityError, Path: itemPath, Message: "identified collection items must be mappings", Primary: primary}}
		}
		mapping := newSequenceMappingIndex(item)
		tuple, locations, missing, entryIndex := mapping.identity(normalized)
		if entryIndex == -1 {
			location := publicLocation(item.Location)
			return nil, []Diagnostic{{Code: codes.invalid, Severity: SeverityError, Path: itemPath.Key(missing), Message: "identified collection item is missing an identity field", Primary: &location}}
		}
		if entryIndex == -2 || entryIndex >= 0 {
			location := publicLocation(item.Location)
			fieldPath := itemPath.Key(missing)
			if entryIndex >= 0 {
				location = publicLocation(item.Mapping[entryIndex].ValueLocation)
				if value := item.Mapping[entryIndex].Value; value != nil {
					location = publicLocation(value.Location)
				}
			}
			return nil, []Diagnostic{{Code: codes.invalid, Severity: SeverityError, Path: fieldPath, Message: "identity fields must be scalar values", Primary: &location}}
		}
		if first, duplicate := firstByTuple[tuple]; duplicate {
			current := publicLocation(locations[0])
			return nil, []Diagnostic{{Code: codes.conflict, Severity: SeverityError, Path: path, Message: "identified collection contains duplicate identities", Primary: ptrLocation(publicLocation(first.location)), Related: []Location{current}}}
		}
		firstByTuple[tuple] = sequenceIdentityRecord{tuple: tuple, index: itemIndex, location: locations[0]}
	}
	return firstByTuple, nil
}

func ptrLocation(location Location) *Location { return &location }