feat(decart): interp

internal/numerics/grid.go

@@ -1,6 +1,9 @@
 package numerics
 
-import "fmt"
+import (
+	"fmt"
+	"math"
+)
 
 // Axis describes a regularly-spaced grid axis with N grid points,
 // values left, left+step, left+2*step, ..., left+(N-1)*step.
@@ -28,27 +31,43 @@ func (e *AxisError) Error() string {
 
 // Bracket holds the two surrounding grid indices and the fractional position
 // of a value within an axis. The weight at Lo is (1 - Frac); the weight at Hi
-// is Frac. Frac lies in [0, 1).
+// is Frac. Frac lies in [0, 1].
 type Bracket struct {
 	Lo, Hi int
 	Frac   float64
 }
 
 // Locate returns the bracket containing value within the axis.
-// For a non-wrapping axis, value must lie in [Left, Left + (N-1)*Step);
-// for a wrapping axis, value must lie in [Left, Left + N*Step).
+// The accepted range is closed at both ends: [Left, Left + (N-1)*Step] for a
+// non-wrapping axis, [Left, Left + N*Step] for a wrapping one.
+//
+// The upper end is closed deliberately. On the GFS latitude axis it is the
+// north pole, whose row holds real data — NCEP resolves it per longitude, so it
+// is a usable grid row like any other. Rejecting it used to abort the wind
+// lookup, and because the caller discarded that error the whole prediction
+// silently froze at latitude 90. The same argument applies to the last forecast
+// hour and the topmost pressure level.
 func (a Axis) Locate(value float64) (Bracket, error) {
 	pos := (value - a.Left) / a.Step
-	lo := int(pos) // truncates toward zero; pos is non-negative for valid inputs
 
 	maxLo := a.N - 2
 	if a.Wrap {
 		maxLo = a.N - 1
 	}
-	if lo < 0 || lo > maxLo {
+	// Bound-check in float space. Checking the truncated index instead would
+	// let values just below Left through: int() truncates toward zero, so a pos
+	// of -0.002 became index 0 rather than -1 and extrapolated off the end.
+	if pos < 0 || pos > float64(maxLo+1) {
 		return Bracket{}, &AxisError{Axis: a.Name, Value: value}
 	}
 
+	lo := int(math.Floor(pos))
+	// The exact upper bound belongs to the top cell at Frac 1, rather than
+	// opening a cell that has no neighbour above it.
+	if lo > maxLo {
+		lo = maxLo
+	}
+
 	hi := lo + 1
 	if a.Wrap && hi == a.N {
 		hi = 0