kube/prototext.libsonnet: implement manifestProtoText

This hairy little ball of recursive object manifestation allows emitting
prototext directly from jsonnet.

Change-Id: I8237c629862cfcdf32ca250fba9eda9926c4f9b4

kube/prototext.libsonnet

@@ -0,0 +1,76 @@
+// An untyped prototext marshaller. Like std.manifestJson, but when you want to
+// emit proto-but-not-really.
+//
+// This supports recursively nested objects/arrays with string, number and bool
+// leaves. All hidden object fields are ignored. The given value must be an
+// object.
+//
+// One unintuitive aspect of proto(text) is that you cannot have
+// multidimensional arrays, ie. the following is forbidden:
+//
+//    manifestProtoText({foo: [[1, 2]]})
+//
+//    RUNTIME ERROR: manifestProtoText: at .foo[0]: double nested arrays are
+//    unsupported by prototext
+
+{
+    local top = self,
+
+    // manifestProtoText takes an object and returns a serialized prototext
+    // representation of it. If indentation is "", then a single-line prototext
+    // will be emitted, otherwise a multi-line prototext will be emitted with
+    // each object depth being indented by the indentation value.
+    manifestProtoText:: function(value, indentation="  ") top.recurse(value, indentation, 0, [""]),
+
+    // Available as std.repeat in jsonnet 0.15
+    repeat:: function(str, count) if count <= 0 then "" else (str + top.repeat(str, count-1)),
+
+    emit:: function(str, indentation, nindent) (
+        top.repeat(indentation, nindent) + str + (if indentation == "" then " " else "\n")
+    ),
+
+    fatal:: function(str, path) error "manifestProtoText: at %s: %s" % [std.join("", path), str],
+
+    // objectField emits a rendered `k: v` object field, potentially recursing
+    // back into recurse for printing the value of object. The value must not
+    // be an array, these are handled/flattened by recurse.
+    objectField:: function(name, value, indentation, nindent, path) (
+        if std.isObject(value) then (
+            top.emit("%s <" % [name], indentation, nindent) +
+            top.recurse(value, indentation, nindent+1, path) +
+            top.emit(">", indentation, nindent)
+        ) else if std.isArray(value) then (
+            top.fatal("double nested arrays are unsupported by prototext", path)
+        ) else if std.isFunction(value) then (
+            top.fatal("cannot manifest functions", path)
+        ) else if std.isBoolean(value) then (
+            top.emit("%s: %s" % [name, if value then "true" else "false"], indentation, nindent)
+        ) else if std.isNumber(value) then (
+            top.emit("%s: %s" % [name, std.toString(value)], indentation, nindent)
+        ) else if std.isString(value) then (
+            // Atempt to properly escape strings via JSON manifestation. Not
+            // entirely sure this always works.
+            top.emit("%s: %s" % [name, std.manifestJson(value)], indentation, nindent)
+        ) else (
+            top.fatal("unknown type %s" % [std.type(value)], path)
+        )
+    ),
+
+    // recurse returns the string representation of an object as prototext.
+    recurse:: function(value, indentation, nindent, path) (
+        if std.isObject(value) then std.join("", [
+            (
+                local field = value[fieldName];
+                if std.isArray(field) then std.join("", std.mapWithIndex(function(i, el) (
+                    local newPath = path + ["."+fieldName, "[%d]" % [i]];
+                    top.objectField(fieldName, el, indentation, nindent, newPath)
+                ), field)) else (
+                    local newPath = path + ["."+fieldName];
+                    top.objectField(fieldName, field, indentation, nindent, newPath)
+                )
+            )
+            for fieldName in std.objectFields(value)
+        ]) else
+        top.fatal("manifestProtoText can only manifest objects", path)
+    ),
+}