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+-- Checks for lib/weather.lua.
+-- Run from the repo root:  lua test/test_weather.lua
+-- The domain tables are what silently misreport when a boundary is off by one,
+-- so every range boundary gets an assertion. The card itself is verified by
+-- screenshot.
+
+package.path = './?.lua;' .. package.path
+local weather = require 'lib.weather'
+
+-- === Condition id to icon =================================================
+-- OWM ids group by hundreds, but the boundaries are not round numbers: the
+-- ranges come from the polybar script that ran against this API for years.
+-- Each assertion pairs the last id in a range with the first id of the next,
+-- which is where an off-by-one would hide.
+-- Sunrise 1500, sunset 1900, so DAY must sit INSIDE that window and NIGHT
+-- outside it. Naming them without checking them against the window is how the
+-- first draft of this test made every timestamp evaluate as night.
+local DAY, NIGHT = 1700, 2000
+
+local function icon(id, now)
+  return weather.icon(id, now, 1500, 1900)   -- sunrise 1500, sunset 1900
+end
+
+-- Thunderstorm: everything up to 232.
+assert(icon(200, DAY) == weather.ICON.thunder_day, 'id 200 day')
+assert(icon(232, DAY) == weather.ICON.thunder_day, 'id 232 is still thunder')
+assert(icon(232, NIGHT) == weather.ICON.thunder_night, 'id 232 night')
+
+-- Light drizzle: 233..311.
+assert(icon(233, DAY) == weather.ICON.drizzle_day, 'id 233 leaves thunder')
+assert(icon(311, DAY) == weather.ICON.drizzle_day, 'id 311 is still light drizzle')
+
+-- Heavy drizzle: 312..321.
+assert(icon(312, DAY) == weather.ICON.drizzle_heavy_day, 'id 312 is heavy drizzle')
+assert(icon(321, DAY) == weather.ICON.drizzle_heavy_day, 'id 321 is still heavy drizzle')
+
+-- Rain: 322..531.
+assert(icon(322, DAY) == weather.ICON.rain_day, 'id 322 is rain')
+assert(icon(531, DAY) == weather.ICON.rain_day, 'id 531 is still rain')
+
+-- Snow: 532..622. One icon, no day/night variant.
+assert(icon(600, DAY) == weather.ICON.snow, 'id 600 is snow')
+assert(icon(622, NIGHT) == weather.ICON.snow, 'snow is the same at night')
+
+-- Fog: 623..771.
+assert(icon(741, DAY) == weather.ICON.fog, 'id 741 is fog')
+assert(icon(771, DAY) == weather.ICON.fog, 'id 771 is still fog')
+
+-- Tornado is a single id, not a range.
+assert(icon(781, DAY) == weather.ICON.tornado, 'id 781 is tornado')
+
+-- Clear and few clouds each have a day and a night face.
+assert(icon(800, DAY) == weather.ICON.clear_day, 'id 800 day is the sun')
+assert(icon(800, NIGHT) == weather.ICON.clear_night, 'id 800 night is the moon')
+assert(icon(801, DAY) == weather.ICON.few_day, 'id 801 day')
+assert(icon(801, NIGHT) == weather.ICON.few_night, 'id 801 night')
+
+-- Overcast: 802..804, no night variant.
+assert(icon(804, DAY) == weather.ICON.overcast, 'id 804 is overcast')
+
+-- Anything outside the known ids must be visibly wrong, not silently sunny.
+assert(icon(999, DAY) == weather.ICON.unknown, 'unknown id gets the error glyph')
+assert(weather.icon(nil, DAY, 1500, 1900) == weather.ICON.unknown, 'nil id')
+
+print('test_weather: all assertions passed')