From 95c906f03dd8fc33b47502739cbe4ce2cb1220dc Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sun, 27 Jun 2021 17:36:25 -0400
Subject: [PATCH] Takes a serious shot at `back_map`.

---
 ClockReceiver/JustInTime.hpp | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/ClockReceiver/JustInTime.hpp b/ClockReceiver/JustInTime.hpp
index c87da1976..32208e5f0 100644
--- a/ClockReceiver/JustInTime.hpp
+++ b/ClockReceiver/JustInTime.hpp
@@ -207,8 +207,22 @@ template <class T, class LocalTimeScale = HalfCycles, int multiplier = 1, int di
 		/// Indicates the amount of time, in the local time scale, until the first local slot that falls wholly
 		/// after @c duration, if that delay were to occur in @c offset units of time from now.
 		[[nodiscard]] forceinline LocalTimeScale back_map(TargetTimeScale duration, TargetTimeScale offset) const {
-			// TODO.
-			return LocalTimeScale(0);
+			// A 1:1 mapping is easy.
+			if constexpr (multiplier == 1 && divider == 1) {
+				return duration;
+			}
+
+			// Work out when this query is placed, and the time to which it relates
+			const auto base = time_since_update_ + offset * divider;
+			const auto target = base + duration * divider;
+
+			// Figure out the number of whole input steps that is required to get
+			// past target, and subtract the number of whole input steps necessary
+			// to get to base.
+			const auto steps_to_base = base.as_integral() / divider;
+			const auto steps_to_target = (target.as_integral() + divider - 1) / divider;
+
+			return LocalTimeScale(steps_to_target - steps_to_base);
 		}
 
 		/// Updates this template's record of the next sequence point.