Function relativePath

find the path to_path, relative to from_path.

TODO: the claim below is wrong, because joinSlash cannot do "./" traversal correctly. for instance joinSlash("a/b/c.txt", "./") === "a/b/c.txt/", but you'd expect it to be "a/b/" if we had correctly resolved the path. which is why the output from this function will not work with joinSlash, but it will work with resolveAsUrl or URL.parse (when you provide from_path as the base path, and from_path is NOT a relative path, otherwise the URL constructor will fail).

if we call the result rel_path, then joining the from_path with rel_path and normalizing it should give you back to_path.

note that both from_path and to_path must have a common root folder in their path strings. for instance, if both paths begin with a relative segment "./", then it will be assumed that both paths are referring to the same common root ancestral directory. however, if for instance, from_path begins with a "C:/" segment, while to_path begins with either "./" or "D:/" or http:// segment, then this function will fail, as it will not be possible for it to navigate/transcend from one point of reference to a completely different point of reference.

so, to be safe, wherever you are certain that both paths are of a certain common type: before passing them here, you should either apply the ensureStartDotSlash function for relative paths, or apply the ensureStartSlash for absolute local paths, or write a custom "ensure" function for your situation. (for example, you could write an "ensureHttp" function that ensures that your path begins with "http").

import { assertEquals, assertThrows } from "jsr:@std/assert"

// aliasing our functions for brevity
const eq = assertEquals, err = assertThrows, fn = relativePath

eq(fn(
	"././hello/world/a/b/c/d/g/../e.txt",
	"././hello/world/a/b/x/y/w/../z/",
), "../../x/y/z/")
eq(fn(
	"././hello/world/a/b/c/d/g/../e.txt",
	"././hello/world/a/b/x/y/w/../z/e.md",
), "../../x/y/z/e.md")
eq(fn(
	".\\./hello\\world\\a/b\\c/d/g/../",
	"././hello/world/a/b/x/y/w/../z/e.md",
), "../../x/y/z/e.md")
eq(fn(
	"././hello/world/a/b/c/d/",
	"././hello/world/a/b/x/y/w/../z/e.md",
), "../../x/y/z/e.md")
eq(fn(
	"././hello/world/a/b/c/d/g/../",
	"././hello/world/a/b/x/y/w/../z/e.md",
), "../../x/y/z/e.md")
eq(fn(
	"././hello/world/a/b/c/d/",
	"././hello/world/a/b/x/y/w/../z/",
), "../../x/y/z/")
eq(fn(
	"./././e.txt",
	"./e.md",
), "./e.md")
eq(fn(
	"/e.txt",
	"/e.md",
), "./e.md")
eq(fn(
	"C:/e.txt",
	"C:/e.md",
), "./e.md")
eq(fn(
	"././hello/world/a/b/c/d/g/../e.txt",
	"././hello/world/a/k/../b/q/../c/d/e.md",
), "./e.md")
eq(fn(
	"./",
	"./",
), "./")
eq(fn(
	"/",
	"/",
), "./")

// there is no common ancestral root between the two paths (since one is absolute, while the other is relative)
err(() => fn(
	"/e.txt",
	"./e.md",
))
// there is no common ancestral root between the two paths
err(() => fn(
	"C:/e.txt",
	"D:/e.md",
))
// there is no common ancestral root between the two paths
err(() => fn(
	"http://e.txt",
	"./e.md",
))
// there is no common ancestral root between the two paths
err(() => fn(
	"file:///C:/e.txt",
	"C:/e.md",
))
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