main_test.go

package main

import (
	"testing"
	"time"
)

func TestCalculateMoonPhase(t *testing.T) {
	tests := []struct {
		name          string
		date          time.Time
		expectedAge   float64
		expectedIllum int
		tolerance     float64
	}{
		{
			name:          "Known new moon",
			date:          time.Date(2000, 1, 6, 18, 14, 0, 0, time.UTC),
			expectedAge:   0.0,
			expectedIllum: 0,
			tolerance:     0.1,
		},
		{
			name:          "Approximately full moon",
			date:          time.Date(2000, 1, 21, 0, 0, 0, 0, time.UTC),
			expectedAge:   14.2,
			expectedIllum: 100,
			tolerance:     1.0,
		},
		{
			name:          "Quarter moon",
			date:          time.Date(2000, 1, 14, 0, 0, 0, 0, time.UTC),
			expectedAge:   7.2,
			expectedIllum: 50,
			tolerance:     1.0,
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			result := calculateMoonPhase(tt.date)

			if abs(result.Age-tt.expectedAge) > tt.tolerance {
				t.Errorf("Age = %.2f, expected %.2f ± %.2f", result.Age, tt.expectedAge, tt.tolerance)
			}

			if abs(float64(result.Illumination-tt.expectedIllum)) > tt.tolerance*10 {
				t.Errorf("Illumination = %d%%, expected %d%% ± %.0f%%", result.Illumination, tt.expectedIllum, tt.tolerance*10)
			}
		})
	}
}

func TestCalculateMoonPhaseRange(t *testing.T) {
	// Test that age is always between 0 and ~29.5 days
	// and illumination is always between 0 and 100
	testDates := []time.Time{
		time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC),
		time.Date(2023, 6, 15, 12, 0, 0, 0, time.UTC),
		time.Date(2024, 12, 31, 23, 59, 59, 0, time.UTC),
	}

	for _, date := range testDates {
		result := calculateMoonPhase(date)

		if result.Age < 0 || result.Age > 30 {
			t.Errorf("Age %.2f is out of valid range [0, 30] for date %s", result.Age, date.Format("2006-01-02"))
		}

		if result.Illumination < 0 || result.Illumination > 100 {
			t.Errorf("Illumination %d%% is out of valid range [0, 100] for date %s", result.Illumination, date.Format("2006-01-02"))
		}
	}
}

func abs(x float64) float64 {
	if x < 0 {
		return -x
	}
	return x
}