Grand test fixup (#138)

* start fixing up tests

* fix up tests + automate with drone

* fiddle with linting

* messing about with drone.yml

* some more fiddling

* hmmm

* add cache

* add vendor directory

* verbose

* ci updates

* update some little things

* update sig

vendor/github.com/golang/geo/s2/nthderivative.go

@@ -0,0 +1,88 @@
+// Copyright 2017 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package s2
+
+// nthDerivativeCoder provides Nth Derivative Coding.
+//   (In signal processing disciplines, this is known as N-th Delta Coding.)
+//
+// Good for varint coding integer sequences with polynomial trends.
+//
+// Instead of coding a sequence of values directly, code its nth-order discrete
+// derivative.  Overflow in integer addition and subtraction makes this a
+// lossless transform.
+//
+//                                       constant     linear      quadratic
+//                                        trend       trend         trend
+//                                      /        \  /        \  /           \_
+// input                               |0  0  0  0  1  2  3  4  9  16  25  36
+// 0th derivative(identity)            |0  0  0  0  1  2  3  4  9  16  25  36
+// 1st derivative(delta coding)        |   0  0  0  1  1  1  1  5   7   9  11
+// 2nd derivative(linear prediction)   |      0  0  1  0  0  0  4   2   2   2
+//                                      -------------------------------------
+//                                      0  1  2  3  4  5  6  7  8   9  10  11
+//                                                  n in sequence
+//
+// Higher-order codings can break even or be detrimental on other sequences.
+//
+//                                           random            oscillating
+//                                      /               \  /                  \_
+// input                               |5  9  6  1   8  8  2 -2   4  -4   6  -6
+// 0th derivative(identity)            |5  9  6  1   8  8  2 -2   4  -4   6  -6
+// 1st derivative(delta coding)        |   4 -3 -5   7  0 -6 -4   6  -8  10 -12
+// 2nd derivative(linear prediction)   |     -7 -2  12 -7 -6  2  10 -14  18 -22
+//                                      ---------------------------------------
+//                                      0  1  2  3  4   5  6  7   8   9  10  11
+//                                                  n in sequence
+//
+// Note that the nth derivative isn't available until sequence item n.  Earlier
+// values are coded at lower order.  For the above table, read 5 4 -7 -2 12 ...
+type nthDerivativeCoder struct {
+	n, m   int
+	memory [10]int32
+}
+
+// newNthDerivativeCoder returns a new coder, where n is the derivative order of the encoder (the N in NthDerivative).
+// n must be within [0,10].
+func newNthDerivativeCoder(n int) *nthDerivativeCoder {
+	c := &nthDerivativeCoder{n: n}
+	if n < 0 || n > len(c.memory) {
+		panic("unsupported n. Must be within [0,10].")
+	}
+	return c
+}
+
+func (c *nthDerivativeCoder) encode(k int32) int32 {
+	for i := 0; i < c.m; i++ {
+		delta := k - c.memory[i]
+		c.memory[i] = k
+		k = delta
+	}
+	if c.m < c.n {
+		c.memory[c.m] = k
+		c.m++
+	}
+	return k
+}
+
+func (c *nthDerivativeCoder) decode(k int32) int32 {
+	if c.m < c.n {
+		c.m++
+	}
+	for i := c.m - 1; i >= 0; i-- {
+		c.memory[i] += k
+		k = c.memory[i]
+	}
+	return k
+}