feat: config and approximate equality (#19)

Establishes a global config object for a TypeDispatcher instance, so far
  with just properties representing comparison tolerances. Begins a
  "relational" group of functions with basic approximate equality, and
  an initial primitive ordering comparison. Ensures that methods that
  depend on properties of `config` will be properly updated when those
  properties change.

Reviewed-on: #19
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>

src/number/relational.js

@@ -0,0 +1,37 @@
+import {onType} from '#core/helpers.js'
+import {Returns} from '#core/Type.js'
+import {Optional} from '#core/TypePatterns.js'
+import {boolnum} from './helpers.js'
+import {NumberT} from './NumberT.js'
+
+// In nanomath, we take the point of view that two comparators are primitive:
+// indistinguishable(a, b, relTol, absTol), and exceeds(a, b). All others
+// are defined generically in terms of these. They typically return BooleanT,
+// but in a numbers-only bundle, they return 1 or 0.
+
+// Notice a feature of TypedDispatcher: if you specify one tolerance, you must
+// specify both.
+export const indistinguishable = onType(
+   [NumberT, NumberT, Optional([NumberT, NumberT])],
+   boolnum((a, b, [tolerances = [0, 0]]) => {
+      const [relTol, absTol] = tolerances
+      if (relTol < 0 || absTol < 0) {
+         throw new RangeError(
+            `Tolerances (relative: ${relTol}, absolute: ${absTol}) `
+            + 'must be nonnegative')
+      }
+      if (isNaN(a) || isNaN(b)) return false
+      if (a === b) return true
+      if (!isFinite(a) || !isFinite(b)) return false
+      // |a-b| <= absTol  or  |a-b| <= relTol*max(|a|, |b|)
+      const diff = Math.abs(a-b)
+      if (diff <= absTol) return true
+      const magnitude = Math.max(Math.abs(a), Math.abs(b))
+      return diff <= relTol * magnitude
+   })
+)
+
+// Returns truthy if a (interpreted as completely precise) represents a
+// greater value than b (interpreted as completely precise). Note that even if
+// so, a and b might be indistinguishable() to some tolerances.
+export const exceeds = onType([NumberT, NumberT], boolnum((a, b) => a > b))