Sorted-key lookup

Coral.AsOf finds the latest key at or before a requested i64 key. Its functions take key tensors or lists and f32 values; they do not take Frame values. Keep the right-hand keys in ascending order. Coral does not check that order. Supply a fallback value for a request before the first right-hand key.

• asof_lookup(keys, values, key, fallback) returns one f32 value.
• asof_join(left_keys, right_keys, right_values, fallback) returns an f32 tensor with one result for each left key.
• asof_lookup_list and asof_join_list offer the same operations on host lists.

module Coral.BookAsOf
import Coral.AsOf (asof_join)
export (main)
def main() -> f32 = {
  left_keys = to_tensor([5i64, 10i64, 25i64, 35i64])
  right_keys = to_tensor([10i64, 20i64, 30i64])
  values = to_tensor([1.0f32, 2.0f32, 3.0f32])
  matches = asof_join(left_keys, right_keys, values, -1.0f32)
  index(to_list(matches), 2i64)
}

The four matches are -1.0, 1.0, 2.0, and 3.0; main returns 2.0.