Initial lookahead for FPGA interchange.

Currently the lookahead is disabled by default because of the time to
compute and RAM usage.  However it does appear to work reasonably well
in testing.  Further effort is required to lower RAM usage after initial
computation, and explore trade-off for cheaper time to compute.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

1 files changed, 174 insertions, 0 deletions

diff --git a/fpga_interchange/sampler.cc b/fpga_interchange/sampler.cc
new file mode 100644
index 00000000..0868867e
--- /dev/null
+++ b/fpga_interchange/sampler.cc
@@ -0,0 +1,174 @@
+/*
+ *  nextpnr -- Next Generation Place and Route
+ *
+ *  Copyright (C) 2021  Symbiflow Authors
+ *
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted, provided that the above
+ *  copyright notice and this permission notice appear in all copies.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+#include "sampler.h"
+#include <algorithm>
+#include <cmath>
+#include <stdexcept>
+
+NEXTPNR_NAMESPACE_BEGIN
+
+static size_t partition_x(std::vector<size_t>::iterator begin, std::vector<size_t>::iterator end,
+                          const std::vector<std::pair<int32_t, int32_t>> &samples)
+{
+    if (std::distance(begin, end) == 0) {
+        return 0;
+    }
+
+    // Find the median x value.
+    std::vector<int32_t> xs;
+    xs.reserve(std::distance(begin, end));
+
+    for (auto iter = begin; iter != end; ++iter) {
+        xs.push_back(samples[*iter].first);
+    }
+
+    std::sort(xs.begin(), xs.end());
+    xs.erase(std::unique(xs.begin(), xs.end()), xs.end());
+
+    // Partion on the median x value (e.g. 50% of samples on one side and
+    // 50% of samples on the other side).
+    int32_t x_div = xs[(xs.size() - 1) / 2];
+
+    auto split = std::partition(begin, end,
+                                [x_div, &samples](size_t index) -> bool { return samples[index].first <= x_div; });
+
+    return std::distance(begin, split);
+}
+
+/* Don't both splitting when the partition has less than kMinSplit. */
+static constexpr ptrdiff_t kMinSplit = 20;
+
+static size_t partition_y(std::vector<size_t>::iterator begin, std::vector<size_t>::iterator end,
+                          const std::vector<std::pair<int32_t, int32_t>> &samples)
+{
+    if (std::distance(begin, end) == 0) {
+        return 0;
+    }
+
+    std::vector<int32_t> ys;
+    ys.reserve(std::distance(begin, end));
+
+    for (auto iter = begin; iter != end; ++iter) {
+        ys.push_back(samples[*iter].second);
+    }
+
+    std::sort(ys.begin(), ys.end());
+    ys.erase(std::unique(ys.begin(), ys.end()), ys.end());
+
+    int32_t y_div = ys[(ys.size() - 1) / 2];
+
+    auto split = std::partition(begin, end,
+                                [y_div, &samples](size_t index) -> bool { return samples[index].second <= y_div; });
+
+    return std::distance(begin, split);
+}
+
+static void add_split(std::vector<size_t> *splits, size_t new_split)
+{
+    if (splits->back() < new_split) {
+        splits->push_back(new_split);
+    } else if (splits->back() != new_split) {
+        throw std::runtime_error("Split is not consectutive!");
+    }
+}
+
+void Sampler::divide_samples(size_t target_sample_count, const std::vector<std::pair<int32_t, int32_t>> &samples)
+{
+    // Initialize indicies lookup and make 1 split with entire sample range.
+    indicies.resize(samples.size());
+    for (size_t i = 0; i < samples.size(); ++i) {
+        indicies[i] = i;
+    }
+
+    splits.reserve(2);
+    splits.push_back(0);
+    splits.push_back(samples.size());
+
+    size_t divisions = std::ceil(std::sqrt(target_sample_count) / 2.);
+    if (divisions == 0) {
+        throw std::runtime_error("Math failure, unreachable!");
+    }
+
+    if (divisions > samples.size()) {
+        // Handle cases where there are few samples.
+        return;
+    }
+
+    // Recursively split samples first 50% / 50% in x direction, and then
+    // 50% / 50% in y direction.  Repeat until the bucket is smaller than
+    // kMinSplit or the samples have been divided `divisions` times.
+    std::vector<size_t> new_splits;
+    for (size_t division_count = 0; division_count < divisions; ++division_count) {
+        new_splits.clear();
+        new_splits.push_back(0);
+        for (size_t i = 0; i < splits.size() - 1; ++i) {
+            size_t split_begin = splits.at(i);
+            size_t split_end = splits.at(i + 1);
+            if (split_end > indicies.size()) {
+                throw std::runtime_error("split_end is not valid!");
+            }
+            if (split_begin >= split_end) {
+                throw std::runtime_error("Invalid split from earlier pass!");
+            }
+
+            std::vector<size_t>::iterator begin = indicies.begin() + split_begin;
+            std::vector<size_t>::iterator end = indicies.begin() + split_end;
+
+            if (std::distance(begin, end) < kMinSplit) {
+                add_split(&new_splits, split_begin);
+                continue;
+            }
+
+            // Try to split samples 50/50 in x direction.
+            size_t split = partition_x(begin, end, samples);
+            // Try to split samples 50/50 in y direction after the x split.
+            size_t split_y1 = partition_y(begin, begin + split, samples);
+            size_t split_y2 = partition_y(begin + split, end, samples);
+
+            // Because the y2 split starts at split, add it here.
+            split_y2 += split;
+
+            add_split(&new_splits, split_begin);
+            add_split(&new_splits, split_begin + split_y1);
+            add_split(&new_splits, split_begin + split);
+            add_split(&new_splits, split_begin + split_y2);
+        }
+
+        add_split(&new_splits, samples.size());
+
+        if (new_splits.front() != 0) {
+            throw std::runtime_error("Split must start at 0");
+        }
+        if (new_splits.back() != samples.size()) {
+            throw std::runtime_error("Split must end at last element");
+        }
+
+        for (size_t i = 0; i < new_splits.size() - 1; ++i) {
+            if (new_splits[i] >= new_splits[i + 1]) {
+                throw std::runtime_error("Split indicies must be increasing");
+            }
+        }
+
+        std::swap(splits, new_splits);
+    }
+}
+
+NEXTPNR_NAMESPACE_END