tests/testthat/test-shift_reference_frame.R

In orbitr: A Tidy Physics Engine for Building and Visualizing Orbital Simulations

test_that("shift_reference_frame centers the target body at origin", {
  sim <- create_system() |>
    add_body("A", mass = 1e30) |>
    add_body("B", mass = 1e24, x = 1e11, vy = 30000) |>
    simulate_system(time_step = 3600, duration = 86400)

  shifted <- shift_reference_frame(sim, "A")

  a_data <- shifted[shifted$id == "A", ]
  expect_true(all(a_data$x == 0))
  expect_true(all(a_data$y == 0))
  expect_true(all(a_data$z == 0))
  expect_true(all(a_data$vx == 0))
  expect_true(all(a_data$vy == 0))
  expect_true(all(a_data$vz == 0))
})

test_that("shift_reference_frame preserves relative distances", {
  sim <- create_system() |>
    add_body("A", mass = 1e30) |>
    add_body("B", mass = 1e24, x = 1e11, vy = 30000) |>
    simulate_system(time_step = 3600, duration = 3600)

  # Distance between A and B at t=0 should be the same before and after shift
  t0_orig <- sim[sim$time == 0, ]
  r_orig <- sqrt(
    (t0_orig$x[t0_orig$id == "A"] - t0_orig$x[t0_orig$id == "B"])^2 +
    (t0_orig$y[t0_orig$id == "A"] - t0_orig$y[t0_orig$id == "B"])^2
  )

  shifted <- shift_reference_frame(sim, "A")
  t0_shift <- shifted[shifted$time == 0, ]
  r_shift <- sqrt(
    (t0_shift$x[t0_shift$id == "A"] - t0_shift$x[t0_shift$id == "B"])^2 +
    (t0_shift$y[t0_shift$id == "A"] - t0_shift$y[t0_shift$id == "B"])^2
  )

  expect_equal(r_orig, r_shift)
})

test_that("shift_reference_frame keep_center = FALSE removes center body", {
  sim <- create_system() |>
    add_body("A", mass = 1e30) |>
    add_body("B", mass = 1e24, x = 1e11, vy = 30000) |>
    simulate_system(time_step = 3600, duration = 3600)

  shifted <- shift_reference_frame(sim, "A", keep_center = FALSE)
  expect_false("A" %in% shifted$id)
  expect_true("B" %in% shifted$id)
})

test_that("shift_reference_frame keep_center = TRUE keeps center body", {
  sim <- create_system() |>
    add_body("A", mass = 1e30) |>
    add_body("B", mass = 1e24, x = 1e11, vy = 30000) |>
    simulate_system(time_step = 3600, duration = 3600)

  shifted <- shift_reference_frame(sim, "A", keep_center = TRUE)
  expect_true("A" %in% shifted$id)
  expect_true("B" %in% shifted$id)
})

test_that("shift_reference_frame errors on unknown body", {
  sim <- create_system() |>
    add_body("A", mass = 1) |>
    simulate_system(time_step = 60, duration = 60)

  expect_error(shift_reference_frame(sim, "Z"), "not found")
})

test_that("shift_reference_frame preserves number of rows", {
  sim <- create_system() |>
    add_body("A", mass = 1e30) |>
    add_body("B", mass = 1e24, x = 1e11, vy = 30000) |>
    simulate_system(time_step = 3600, duration = 86400)

  shifted <- shift_reference_frame(sim, "A")
  expect_equal(nrow(shifted), nrow(sim))
})

test_that("shift_reference_frame to the barycenter zeros the center of mass", {
  sim <- create_system() |>
    add_body("A", mass = 2e30, x = 1e11, vy = 10000) |>
    add_body("B", mass = 1e30, x = -1e11, vy = -30000) |>
    simulate_system(time_step = 3600, duration = 86400 * 5)

  shifted <- shift_reference_frame(sim, "barycenter")
  com <- shifted |>
    dplyr::group_by(time) |>
    dplyr::summarise(x = sum(mass * x) / sum(mass),
                     vy = sum(mass * vy) / sum(mass))

  expect_equal(nrow(shifted), nrow(sim))
  expect_lt(max(abs(com$x)), 1e-3)
  expect_lt(max(abs(com$vy)), 1e-9)

  # relative separation is unchanged
  sep <- function(d) {
    t0 <- d[d$time == 0, ]
    abs(t0$x[t0$id == "A"] - t0$x[t0$id == "B"])
  }
  expect_equal(sep(sim), sep(shifted))
})

test_that("a body literally named 'barycenter' is treated as a body", {
  sim <- create_system() |>
    add_body("barycenter", mass = 1e30) |>
    add_body("B", mass = 1e24, x = 1e11, vy = 30000) |>
    simulate_system(time_step = 3600, duration = 3600)

  shifted <- shift_reference_frame(sim, "barycenter")
  expect_true(all(shifted$x[shifted$id == "barycenter"] == 0))
})