package generatedtext

import (
	"bytes"
	"encoding/json"
	"fmt"
	"unicode/utf8"
)

const (
	// MaxGeneratedTextBytes is the largest provider response accepted for one
	// generated-text report before JSON decoding begins.
	MaxGeneratedTextBytes = 64 * 1024

	maxGeneratedTextCharacters          = 20_000
	maxGeneratedTextStringCharacters    = 4_000
	maxHourlyDiscussionCharacters       = 12_000
	maxDayStyleDiscussionParagraphs     = 12
	maxDayStyleDiscussionParagraphChars = 4_000
)

// ValidateRawOutput rejects generated JSON before it is decoded or retained.
func ValidateRawOutput(data []byte) error {
	if len(data) > MaxGeneratedTextBytes {
		return fmt.Errorf("generated text exceeds the %d-byte limit", MaxGeneratedTextBytes)
	}
	return nil
}

func validateGeneratedTextSize(data []byte, name string) error {
	if err := ValidateRawOutput(data); err != nil {
		return fmt.Errorf("%s %w", name, err)
	}
	return nil
}

func validateGeneratedTextField(name, field string, raw json.RawMessage, total *int) error {
	switch field {
	case "summary", "precipitation_timing":
		return validateGeneratedTextString(name, field, raw, maxGeneratedTextStringCharacters, total)
	case "forecast_discussion":
		if name == "hourly" {
			return validateGeneratedTextString(name, field, raw, maxHourlyDiscussionCharacters, total)
		}
		return validateDayStyleDiscussion(name, raw, total)
	default:
		return fmt.Errorf("decode %s generated text: unsupported field", name)
	}
}

func validateGeneratedTextString(name, field string, raw json.RawMessage, limit int, total *int) error {
	if bytes.Equal(bytes.TrimSpace(raw), []byte("null")) {
		return fmt.Errorf("%s generated text %s must be a string", name, generatedTextFieldLabel(field))
	}
	var value string
	if err := json.Unmarshal(raw, &value); err != nil {
		return fmt.Errorf("%s generated text %s must be a string", name, generatedTextFieldLabel(field))
	}
	characters := utf8.RuneCountInString(value)
	if characters > limit {
		return fmt.Errorf("%s generated text %s exceeds the %d-character limit", name, generatedTextFieldLabel(field), limit)
	}
	return addGeneratedTextCharacters(name, characters, total)
}

func validateDayStyleDiscussion(name string, raw json.RawMessage, total *int) error {
	decoder := json.NewDecoder(bytes.NewReader(raw))
	token, err := decoder.Token()
	if err != nil {
		return fmt.Errorf("%s generated text forecast discussion must be an array", name)
	}
	if delimiter, ok := token.(json.Delim); !ok || delimiter != '[' {
		return fmt.Errorf("%s generated text forecast discussion must be an array", name)
	}

	paragraphs := 0
	for decoder.More() {
		paragraphs++
		if paragraphs > maxDayStyleDiscussionParagraphs {
			return fmt.Errorf("%s generated text forecast discussion exceeds the %d-paragraph limit", name, maxDayStyleDiscussionParagraphs)
		}
		var rawParagraph json.RawMessage
		if err := decoder.Decode(&rawParagraph); err != nil || bytes.Equal(bytes.TrimSpace(rawParagraph), []byte("null")) {
			return fmt.Errorf("%s generated text forecast discussion paragraphs must be strings", name)
		}
		var paragraph string
		if err := json.Unmarshal(rawParagraph, &paragraph); err != nil {
			return fmt.Errorf("%s generated text forecast discussion paragraphs must be strings", name)
		}
		characters := utf8.RuneCountInString(paragraph)
		if characters > maxDayStyleDiscussionParagraphChars {
			return fmt.Errorf("%s generated text forecast discussion paragraphs exceed the %d-character limit", name, maxDayStyleDiscussionParagraphChars)
		}
		if err := addGeneratedTextCharacters(name, characters, total); err != nil {
			return err
		}
	}
	if _, err := decoder.Token(); err != nil {
		return fmt.Errorf("%s generated text forecast discussion must be an array", name)
	}
	return nil
}

func addGeneratedTextCharacters(name string, characters int, total *int) error {
	*total += characters
	if *total > maxGeneratedTextCharacters {
		return fmt.Errorf("%s generated text exceeds the %d-character limit", name, maxGeneratedTextCharacters)
	}
	return nil
}

func generatedTextFieldLabel(field string) string {
	if field == "forecast_discussion" {
		return "forecast discussion"
	}
	if field == "precipitation_timing" {
		return "precipitation timing"
	}
	return field
}