// Package sqllex recognizes PostgreSQL lexical boundaries without parsing SQL.
package sqllex

import "strings"

func start(c byte) bool { return c >= 128 || c == '_' || c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' }
func ident(c byte) bool { return start(c) || c >= '0' && c <= '9' || c >= 128 }

// Walk visits named parameters and standalone line comments in executable SQL. Quoted
// strings/identifiers, dollar bodies, casts and nested block comments are opaque.
// Returning replacement text is literal: it is never scanned again.
func Walk(sql string, parameter, lineComment func(string) string) string {
	var out strings.Builder
	for i := 0; i < len(sql); {
		begin := i
		switch {
		case strings.HasPrefix(sql[i:], "--"):
			i += 2
			for i < len(sql) && sql[i] != '\n' && sql[i] != '\r' {
				i++
			}
			text := sql[begin:i]
			if lineComment != nil && strings.TrimSpace(sql[strings.LastIndexAny(sql[:begin], "\r\n")+1:begin]) == "" {
				text = lineComment(text)
			}
			out.WriteString(text)
			continue
		case strings.HasPrefix(sql[i:], "/*"):
			i += 2
			depth := 1
			for i < len(sql) && depth > 0 {
				if strings.HasPrefix(sql[i:], "/*") {
					depth++
					i += 2
				} else if strings.HasPrefix(sql[i:], "*/") {
					depth--
					i += 2
				} else {
					i++
				}
			}
		case sql[i] == '\'' || sql[i] == '"':
			q := sql[i]
			escapes := q == '\'' && i > 0 && (sql[i-1] == 'E' || sql[i-1] == 'e') && (i == 1 || !ident(sql[i-2]))
			i++
			for i < len(sql) {
				if escapes && sql[i] == '\\' {
					i++
					if i < len(sql) {
						i++
					}
					continue
				}
				if sql[i] == q {
					i++
					if i < len(sql) && sql[i] == q {
						i++
						continue
					}
					break
				}
				i++
			}
		case sql[i] == '$' && (i == 0 || !ident(sql[i-1]) && sql[i-1] != '$'):
			j := i + 1
			if j < len(sql) && start(sql[j]) {
				for j < len(sql) && ident(sql[j]) {
					j++
				}
			}
			if j < len(sql) && sql[j] == '$' {
				tag := sql[i : j+1]
				end := strings.Index(sql[j+1:], tag)
				if end < 0 {
					i = len(sql)
				} else {
					i = j + 1 + end + len(tag)
				}
			} else {
				i++
			}
		case strings.HasPrefix(sql[i:], "::"):
			i += 2
		case sql[i] == ':' && i+1 < len(sql) && start(sql[i+1]) && (i == 0 || !ident(sql[i-1]) && sql[i-1] != ':'):
			i += 2
			for i < len(sql) && ident(sql[i]) {
				i++
			}
			text := sql[begin:i]
			if parameter != nil {
				text = parameter(text)
			}
			out.WriteString(text)
			continue
		default:
			i++
		}
		out.WriteString(sql[begin:i])
	}
	return out.String()
}