package gfs

import "fmt"

// Dataset shape: (hour, pressure_level, variable, latitude, longitude).
// Matches the cube layout used by the reference Tawhiri implementation.
const (
	NumHours      = 65  // 0, 3, 6, ..., 192 hours forecast
	NumLevels     = 47  // pressure levels
	NumVariables  = 3   // geopotential height, U-wind, V-wind
	NumLatitudes  = 361 // -90.0 to +90.0 inclusive in 0.5° steps
	NumLongitudes = 720 // 0.0 to 359.5 in 0.5° steps

	HourStep   = 3
	MaxHour    = 192
	Resolution = 0.5
	LatStart   = -90.0
	LonStart   = 0.0

	VarHeight = 0
	VarWindU  = 1
	VarWindV  = 2

	ElementSize = 4 // float32

	// DatasetSize is the canonical file size: every grid cell × element size.
	DatasetSize int64 = int64(NumHours) * int64(NumLevels) * int64(NumVariables) *
		int64(NumLatitudes) * int64(NumLongitudes) * int64(ElementSize)
)

// LevelSet identifies which GRIB file (primary/secondary) carries a level.
type LevelSet int

const (
	LevelSetA LevelSet = iota // pgrb2 — primary file
	LevelSetB                 // pgrb2b — secondary file
)

// Pressures lists the 47 pressure levels (hPa) in dataset index order,
// descending from surface to top of atmosphere.
var Pressures = [NumLevels]int{
	1000, 975, 950, 925, 900, 875, 850, 825, 800, 775,
	750, 725, 700, 675, 650, 625, 600, 575, 550, 525,
	500, 475, 450, 425, 400, 375, 350, 325, 300, 275,
	250, 225, 200, 175, 150, 125, 100, 70, 50, 30,
	20, 10, 7, 5, 3, 2, 1,
}

// PressuresPgrb2 lists the levels carried by the primary GRIB file.
var PressuresPgrb2 = []int{
	10, 20, 30, 50, 70, 100, 150, 200, 250, 300, 350, 400,
	450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 925,
	950, 975, 1000,
}

// PressuresPgrb2b lists the levels carried by the secondary GRIB file.
var PressuresPgrb2b = []int{
	1, 2, 3, 5, 7, 125, 175, 225, 275, 325, 375, 425,
	475, 525, 575, 625, 675, 725, 775, 825, 875,
}

var pressureIndex map[int]int
var pressureLevelSet map[int]LevelSet

func init() {
	pressureIndex = make(map[int]int, NumLevels)
	for i, p := range Pressures {
		pressureIndex[p] = i
	}
	pressureLevelSet = make(map[int]LevelSet, NumLevels)
	for _, p := range PressuresPgrb2 {
		pressureLevelSet[p] = LevelSetA
	}
	for _, p := range PressuresPgrb2b {
		pressureLevelSet[p] = LevelSetB
	}
}

// PressureIndex returns the dataset index for a pressure level in hPa,
// or -1 when the level is unknown.
func PressureIndex(hPa int) int {
	idx, ok := pressureIndex[hPa]
	if !ok {
		return -1
	}
	return idx
}

// PressureLevelSet returns the GRIB file set carrying a pressure level.
func PressureLevelSet(hPa int) (LevelSet, bool) {
	ls, ok := pressureLevelSet[hPa]
	return ls, ok
}

// HourIndex returns the dataset time index for a forecast hour, or -1 when
// the hour is outside the range or not a multiple of HourStep.
func HourIndex(hour int) int {
	if hour < 0 || hour > MaxHour || hour%HourStep != 0 {
		return -1
	}
	return hour / HourStep
}

// Hours returns the full list of forecast hours, [0, 3, 6, ..., MaxHour].
func Hours() []int {
	out := make([]int, 0, NumHours)
	for h := 0; h <= MaxHour; h += HourStep {
		out = append(out, h)
	}
	return out
}

// VariableIndex maps a GRIB (category, number) pair to a dataset variable
// index, returning -1 for parameters this dataset does not store.
func VariableIndex(parameterCategory, parameterNumber int) int {
	switch {
	case parameterCategory == 3 && parameterNumber == 5:
		return VarHeight
	case parameterCategory == 2 && parameterNumber == 2:
		return VarWindU
	case parameterCategory == 2 && parameterNumber == 3:
		return VarWindV
	default:
		return -1
	}
}

// S3 URL configuration for NOAA GFS data on the public S3 mirror.
const S3BaseURL = "https://noaa-gfs-bdp-pds.s3.amazonaws.com"

// GribURL returns the S3 URL for a primary (pgrb2) GRIB file.
func GribURL(date string, runHour, forecastStep int) string {
	return fmt.Sprintf("%s/gfs.%s/%02d/atmos/gfs.t%02dz.pgrb2.0p50.f%03d",
		S3BaseURL, date, runHour, runHour, forecastStep)
}

// GribURLB returns the S3 URL for a secondary (pgrb2b) GRIB file.
func GribURLB(date string, runHour, forecastStep int) string {
	return fmt.Sprintf("%s/gfs.%s/%02d/atmos/gfs.t%02dz.pgrb2b.0p50.f%03d",
		S3BaseURL, date, runHour, runHour, forecastStep)
}