// Package classify maps a DNS hostname to a cloud/database/storage resource.
//
// The hot path is deliberately allocation-free: it never lowercases unless an
// uppercase byte is present, never splits the hostname into a []string, never
// uses regexp/JSON/reflection, and returns substrings of the input as the
// region/resource. See BenchmarkClassify* for the 0 allocs/op assertion.
package classify

import (
	"strings"

	"tea.karlcom.de/karl/bucketeer/internal/model"
)

const maxLabels = 64

// scanner holds the byte offsets of each dot-separated label of a hostname.
// It is stack allocated by Classify and never heap allocated.
type scanner struct {
	host  string
	n     int
	start [maxLabels]int32
}

func newScanner(host string) scanner {
	var s scanner
	s.host = host
	start := int32(0)
	for i := 0; i < len(host); i++ {
		if host[i] == '.' {
			if s.n >= maxLabels {
				s.n = -1
				return s
			}
			s.start[s.n] = start
			s.n++
			start = int32(i + 1)
		}
	}
	if s.n >= maxLabels {
		s.n = -1
		return s
	}
	s.start[s.n] = start
	s.n++
	return s
}

// label returns the i-th label (0 = leftmost). Callers must ensure i < n.
func (s *scanner) label(i int) string {
	end := int32(len(s.host))
	if i+1 < s.n {
		end = s.start[i+1] - 1
	}
	return s.host[s.start[i]:end]
}

func hasUpper(s string) bool {
	for i := 0; i < len(s); i++ {
		if s[i] >= 'A' && s[i] <= 'Z' {
			return true
		}
	}
	return false
}

func trimTrailingDots(s string) string {
	for len(s) > 0 && s[len(s)-1] == '.' {
		s = s[:len(s)-1]
	}
	return s
}

func validHost(host string) bool {
	if host == "" || len(host) > 253 {
		return false
	}
	for i := 0; i < len(host); i++ {
		c := host[i]
		switch {
		case c >= 'a' && c <= 'z':
		case c >= '0' && c <= '9':
		case c == '-' || c == '.' || c == '_':
		default:
			return false
		}
	}
	return true
}

// Classify returns the provider resource described by host, if any.
// The returned Match's string fields alias host.
func Classify(host string) (model.Match, bool) {
	host = trimTrailingDots(host)
	if host == "" || len(host) > 253 {
		return model.Match{}, false
	}
	// Wildcard certificates never describe a concrete resource.
	if host[0] == '*' {
		return model.Match{}, false
	}
	if hasUpper(host) {
		host = strings.ToLower(host)
	}
	if !validHost(host) {
		return model.Match{}, false
	}

	s := newScanner(host)
	if s.n < 2 {
		return model.Match{}, false
	}

	switch {
	case strings.HasSuffix(host, ".amazonaws.com"):
		return classifyAWS(&s)
	case strings.HasSuffix(host, ".core.windows.net"):
		return classifyAzureCore(&s)
	case strings.HasSuffix(host, ".database.windows.net"):
		return classifyAzureDatabase(&s)
	case strings.HasSuffix(host, ".database.azure.com"):
		return classifyAzureFlexibleServer(&s)
	case strings.HasSuffix(host, ".documents.azure.com"):
		return classifyAzureDocuments(&s)
	case strings.HasSuffix(host, ".cosmos.azure.com"):
		return classifyAzureCosmos(&s)
	case strings.HasSuffix(host, ".cache.windows.net"):
		return classifyAzureRedis(&s)
	case strings.HasSuffix(host, ".redis.azure.net"):
		return classifyAzureManagedRedis(&s)
	case strings.HasSuffix(host, ".search.windows.net"):
		return classifyAzureSearch(&s)
	case strings.HasSuffix(host, ".storage.googleapis.com"):
		return classifyGCS(&s)
	case strings.HasSuffix(host, ".firebaseio.com"):
		return classifyFirebaseIO(&s)
	case strings.HasSuffix(host, ".firebasedatabase.app"):
		return classifyFirebaseDB(&s)
	case strings.HasSuffix(host, ".appspot.com"):
		return classifyAppspot(&s)
	case strings.HasSuffix(host, ".r2.cloudflarestorage.com"):
		return classifyR2(&s)
	case strings.HasSuffix(host, ".r2.dev"):
		return classifyR2Dev(&s)
	case strings.HasSuffix(host, ".digitaloceanspaces.com"):
		return classifyDigitalOceanSpaces(&s)
	case strings.HasSuffix(host, ".ondigitalocean.com"):
		return classifyDigitalOceanDB(&s)
	case strings.HasSuffix(host, ".supabase.co"):
		return classifySupabase(&s)
	case strings.HasSuffix(host, ".supabase.com"):
		return classifySupabasePooler(&s)
	case strings.HasSuffix(host, ".customer-oci.com"):
		return classifyOCIResource(&s)
	case strings.HasSuffix(host, ".oraclecloud.com"):
		return classifyOCIGeneric(&s)
	}
	return model.Match{}, false
}

// ShouldEnrich reports whether a non-matching hostname is worth a DNS/CNAME
// lookup. It rejects wildcards, the certspotter unparsable placeholder, single
// labels and obvious provider service apexes.
func ShouldEnrich(host string) bool {
	host = trimTrailingDots(host)
	if len(host) < 3 || host[0] == '*' {
		return false
	}
	if host[0] == '<' { // certspotter's "<unparsable>" placeholder
		return false
	}
	if !strings.Contains(host, ".") {
		return false
	}
	if hasUpper(host) {
		host = strings.ToLower(host)
	}
	// Provider service apexes never yield a customer resource via CNAME.
	switch {
	case host == "amazonaws.com", strings.HasSuffix(host, ".amazonaws.com"):
		return false
	case host == "core.windows.net", strings.HasSuffix(host, ".windows.net"):
		return false
	case host == "azure.com", strings.HasSuffix(host, ".azure.com"):
		return false
	case host == "googleapis.com", strings.HasSuffix(host, ".googleapis.com"):
		return false
	case host == "cloudflarestorage.com", host == "r2.dev":
		return false
	case strings.HasSuffix(host, ".digitaloceanspaces.com"):
		return false
	case strings.HasSuffix(host, ".supabase.co"), strings.HasSuffix(host, ".supabase.com"):
		return false
	case strings.HasSuffix(host, ".oraclecloud.com"), strings.HasSuffix(host, ".customer-oci.com"):
		return false
	}
	return true
}