Calculating the angle formed by three points in PICO-8

angletest.lua

-- ❎ changes selected point
-- direction keys move point

function _init()
 a = {x=20,y=20}
 b = {x=80,y=80}
 c = {x=40,y=60}
 pts = {a,b,c}
 sel = 1
end

function _update()
 if (btn(⬆️) and pts[sel].y > 0) pts[sel].y -= 1
 if (btn(⬇️) and pts[sel].y < 127) pts[sel].y += 1
 if (btn(⬅️) and pts[sel].x > 0) pts[sel].x -= 1
 if (btn(➡️) and pts[sel].x < 127) pts[sel].x += 1
 if (btnp(❎)) sel = ((sel + 1) % #pts) + 1
end

function _draw()
 cls()
 line(a.x, a.y, b.x, b.y, 13)
 line(b.x, b.y, c.x, c.y, 14)
 circfill(a.x, a.y, 2, 8)
 circfill(b.x, b.y, 2, 11)
 circfill(c.x, c.y, 2, 12)
 circ(pts[sel].x, pts[sel].y, 4, 7)

 ab = atan2(b.x-a.x, b.y-a.y)
 bc = atan2(b.x-c.x, b.y-c.y)
 angle = ab-bc
 print('\f8a\fbb\f6='..ab, 0, 128-18, 6)
 print('\fcc\fbb\f6='..bc, 0, 128-12, 6)
 print('angle='..angle, 0, 128-6, 6)
end