step one

internal/weather/gfs/constants.go

@@ -0,0 +1,141 @@
+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)
+}