feat: add cadence methods

benchmark/cadence.jl

@@ -0,0 +1,45 @@
+using Chairmarks
+using Dates
+using SpaceDataModel.Times
+
+function slow_resolution(times; rtol = 1.0e-3, check = true)
+    dts = diff(times)
+    check && (all(d -> ≃(d, dts[1]; rtol), dts) || error("Data is not approximately uniformly sampled."))
+    return dts[1]
+end
+
+# Generate large approximately sampled times with small noise
+function generate_approx_times(n, dt, noise_level = 1.0e-5)
+    base = range(0.0, step = dt, length = n)
+    return collect(base) .+ noise_level .* randn(n)
+end
+
+function generate_approx_times2(n, dt, noise_level = 1.0e-5)
+    base = range(0.0, step = dt, length = n)
+    return 1.0e9 .* Nanosecond.(collect(base)) .+ round.(Int64, noise_level .* randn(n) .* 1.0e9) .* Nanosecond(1)
+end
+
+# Benchmark suite
+function run_resolution_benchmarks()
+    sizes = [10^3, 10^4, 10^5, 10^6]
+    dt = 1.0
+    println("Resolution Benchmark")
+    println("="^60)
+
+    for n in sizes
+        times_range = range(0.0, step = dt, length = n)
+        for f in [generate_approx_times, generate_approx_times2]
+            times = f(n, dt)
+            b_range = @b cadence($times_range) samples = 10 evals = 3
+            b_check = @b cadence($times; check = true) samples = 10 evals = 3
+            b_check2 = @b slow_resolution($times; check = true) samples = 10 evals = 3
+            println("\nn = $(n):")
+            @info "b_check" b_check b_check2 b_range
+        end
+    end
+
+    return
+end
+
+
+run_resolution_benchmarks()

src/SpaceDataModel.jl

@@ -27,4 +27,7 @@ include("workload.jl")
 
 include("variable_interface.jl")
 
+include("times.jl")
+using .Times
+
 end

src/times.jl

@@ -0,0 +1,38 @@
+# Reference: https://github.com/JuliaAPlavin/DateFormats.jl
+module Times
+using Dates
+using Dates: AbstractTime
+export ≃, cadence
+
+/ₜ(x, n) = x / n
+/ₜ(x::AbstractTime, n) = Nanosecond(round(Int64, Dates.tons(x) / n))
+
+# workaround for `no method matching isapprox(::Nanosecond, ::Nanosecond)`
+≃(x, y; kw...) = isapprox(x, y; kw...)
+≃(x::AbstractTime, y; kw...) = isapprox(Dates.tons(x), Dates.tons(y); kw...)
+
+"""
+    cadence(times; rtol=1.0e-3, check=true)
+    cadence(T<:Real, times; rtol=1.0e-3, check=true)
+
+Return the time step of uniformly sampled `times`.
+
+If `check=true`, validates that samples are approximately uniform within `rtol`.
+Pass a type `T<:Real` to return the cadence in seconds as that type.
+"""
+cadence(times::AbstractRange) = step(times)
+function cadence(times; rtol = 1.0e-3, check = true)
+    N = length(times)
+    @assert N > 1
+    dt0 = /ₜ(times[N] - times[1], N - 1)
+    check && @inbounds for i in 1:(N - 1)
+        dt = times[i + 1] - times[i]
+        @assert ≃(dt, dt0; rtol) "Data is not approximately uniformly sampled."
+    end
+    return dt0
+end
+function cadence(T::Type{<:Real}, times; kw...)
+    dt = cadence(times; kw...)
+    return dt isa AbstractTime ? T(Dates.tons(dt) / 1.0e9) : T(dt)
+end
+end

test/Project.toml

@@ -2,6 +2,7 @@
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
 CheckConcreteStructs = "73c92db5-9da6-4938-911a-6443a7e94a58"
 CommonDataModel = "1fbeeb36-5f17-413c-809b-666fb144f157"
+Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
 DimensionalData = "0703355e-b756-11e9-17c0-8b28908087d0"
 JET = "c3a54625-cd67-489e-a8e7-0a5a0ff4e31b"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

test/times.jl

@@ -0,0 +1,31 @@
+@testitem "cadence" begin
+    using Dates
+    using SpaceDataModel: cadence, ≃
+
+    # AbstractRange - returns step directly
+    @test cadence(1:10) == 1
+    @test cadence(0.0:0.5:10.0) == 0.5
+    @test cadence(Nanosecond(1):Nanosecond(1):Nanosecond(100)) == Nanosecond(1)
+
+    # Vector{Float64}
+    times = collect(0.0:1.0:100.0)
+    @test cadence(times) == 1.0
+    @test cadence(Float64, times) == 1.0
+
+    # Approximately uniform sampling
+    times_noisy = times .+ 1.0e-6 .* randn(length(times))
+    @test ≃(cadence(times_noisy), 1.0; rtol = 1.0e-3)
+
+    # Non-uniform sampling should error
+    times_bad = [0.0, 1.0, 3.0, 4.0]
+    @test_throws AssertionError cadence(times_bad)
+    @test cadence(times_bad; check = false) ≈ 4.0 / 3
+
+    # Dates support
+    dt_range = DateTime(2020):Hour(1):DateTime(2020, 1, 2)
+    @test cadence(dt_range) == Hour(1)
+
+    ns_times = Nanosecond.(0:1_000_000_000:10_000_000_000)
+    @test cadence(ns_times) == Nanosecond(1_000_000_000)
+    @test cadence(Float64, ns_times) == 1.0
+end