d2compiler: fix underscore links

ci/release/changelogs/next.md

@@ -14,3 +14,4 @@
 - Edge globs setting styles inherit correctly in child boards [#1967](https://github.com/terrastruct/d2/pull/1967)
 - Board links imported with spread imports work [#1972](https://github.com/terrastruct/d2/pull/1972)
 - Fix importing a file with nested boards [#1998](https://github.com/terrastruct/d2/pull/1998)
+- Fix importing a file with underscores in links [#1999](https://github.com/terrastruct/d2/pull/1999)

d2compiler/compile_test.go

@@ -2917,6 +2917,39 @@ layers: {
   }
 }`,
 			},
+			assertions: func(t *testing.T, g *d2graph.Graph) {
+				tassert.Equal(t, "root.layers.x.layers.b", g.Layers[0].Objects[0].Link.Value)
+			},
+		},
+		{
+			name: "import-link-underscore",
+			text: `k
+
+layers: {
+  x: {...@x}
+}`,
+			files: map[string]string{
+				"x.d2": `a
+layers: {
+  b: {
+    d.link: _
+
+    layers: {
+      c: {
+        c.link: _
+				z.link: _._
+				f.link: _._.layers.b
+      }
+    }
+  }
+}`,
+			},
+			assertions: func(t *testing.T, g *d2graph.Graph) {
+				tassert.Equal(t, "root.layers.x", g.Layers[0].Layers[0].Objects[0].Link.Value)
+				tassert.Equal(t, "root.layers.x.layers.b", g.Layers[0].Layers[0].Layers[0].Objects[0].Link.Value)
+				tassert.Equal(t, "root.layers.x", g.Layers[0].Layers[0].Layers[0].Objects[1].Link.Value)
+				tassert.Equal(t, "root.layers.x.layers.b", g.Layers[0].Layers[0].Layers[0].Objects[2].Link.Value)
+			},
 		},
 		{
 			name: "import-nested-layers",

d2ir/compile.go

@@ -863,8 +863,17 @@ func (c *compiler) updateLinks(m *Map) {
 			}
 			bida := BoardIDA(f)
 			aida := IDA(f)
+
+			// The id path from that board to field
+			relaida := aida[len(bida)+1 : len(aida)-len(bida)]
+			// If the link value has underscores, the path length can be less than the path length of the field
+			uplevels := len(relaida) - len(linkIDA) + 1
+
 			if len(bida) != len(aida) {
 				prependIDA := aida[:len(aida)-len(bida)]
+				if uplevels > 0 {
+					prependIDA = prependIDA[:len(prependIDA)-uplevels]
+				}
 				fullIDA := []string{"root"}
 				// With nested imports, a value may already have been updated with part of the absolute path
 				// E.g.,
@@ -876,9 +885,11 @@ func (c *compiler) updateLinks(m *Map) {
 				// -------
 				// a b c d
 			OUTER:
+				// Starts at 1 assuming 0 is "root" for both
+				// +2 assuming layers/scenarios/steps is in between both
 				for i := 1; i < len(prependIDA); i += 2 {
 					for j := 0; i+j < len(prependIDA); j++ {
-						if prependIDA[i+j] != linkIDA[1+j] {
+						if 1+j >= len(linkIDA) || prependIDA[i+j] != linkIDA[1+j] {
 							break
 						}
 						// Reached the end and all common