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diff --git a/goldfishterm/internal/string_capability.h b/goldfishterm/internal/string_capability.h
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+// Copyright 2021 Benjamin Barenblat
+//
+// 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
+//
+//     https://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.
+
+// Parses and interprets terminfo string capabilities. These are written in a
+// simple stack-based programming language whose semantics are documented in
+// terminfo(5).
+
+#ifndef EC_GOLDFISHTERM_INTERNAL_STRING_CAPABILITY_H_
+#define EC_GOLDFISHTERM_INTERNAL_STRING_CAPABILITY_H_
+
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "goldfishterm/internal/emit.h"
+#include "goldfishterm/terminfo.h"
+#include "third_party/abseil/absl/types/optional.h"
+#include "third_party/abseil/absl/types/variant.h"
+
+namespace goldfishterm_internal {
+
+using StringCapabilityParameter = absl::variant<int, std::string>;
+
+struct InterpretStringCapabilityInput {
+  std::string capability;
+  std::vector<StringCapabilityParameter> parameters;
+
+  // The number of lines affected by this operation.
+  int lines_affected;
+
+  // The minimum baud rate at which padding should be expanded.
+  int padding_baud_rate;
+
+  // The padding character, or absl::nullopt if the terminal doesn't support
+  // padding.
+  absl::optional<char> pad_char;
+
+  // True iff the terminal supports XON/XOFF handshaking.
+  bool has_xon_xoff;
+
+  // The baud rate of the terminal.
+  int baud;
+
+  // The number of extra bits sent per character. For a terminal emulator, this
+  // is generally 0; for an RS-232 line, it's generally 2 or 3 (one start bit,
+  // one stop bit, and possibly one parity bit).
+  int extra_bits_per_character;
+
+  // Constructs an InterpretStringCapabilityInput with fail-safe defaults.
+  explicit InterpretStringCapabilityInput() noexcept
+      : lines_affected(1),
+        padding_baud_rate(0),
+        pad_char('\0'),
+        has_xon_xoff(false),
+        baud(9600),
+        extra_bits_per_character(0) {}
+
+  // Constructs an InterpretStringCapabilityInput from a terminfo entry.
+  explicit InterpretStringCapabilityInput(
+      const goldfishterm::TerminfoEntry& terminfo, std::string capability_,
+      std::vector<StringCapabilityParameter> parameters_, int baud_,
+      int lines_affected_) noexcept
+      : capability(std::move(capability_)),
+        parameters(std::move(parameters_)),
+        lines_affected(lines_affected_),
+        padding_baud_rate(
+            terminfo.get(goldfishterm::NumericCapability::kPaddingBaudRate)
+                .value_or(0)),
+        pad_char(terminfo.get(goldfishterm::StringCapability::kCharPadding)
+                     .value_or(std::string(1, '\0'))[0]),
+        has_xon_xoff(terminfo.get(goldfishterm::BooleanCapability::kXonXoff)),
+        baud(baud_),
+        extra_bits_per_character(0) {}
+};
+
+struct InterpretStringCapabilityResult {
+  // Terms in the emitter EDSL (see emit.h).
+  std::vector<std::shared_ptr<const EmitTerm>> terms;
+
+  // The expected cost of emitting these terms. This is a unitless, arbitrarily
+  // scaled real number; it will be at least 0, but you should not rely on any
+  // absolute semantics.
+  float cost;
+};
+
+// Interprets a terminfo string capability, expanding parameters and inserting
+// padding and delays as appropriate. Throws std::runtime_error if parsing or
+// interpretation fails.
+InterpretStringCapabilityResult InterpretStringCapability(
+    const InterpretStringCapabilityInput&);
+
+}  // namespace goldfishterm_internal
+
+#endif  // EC_GOLDFISHTERM_INTERNAL_STRING_CAPABILITY_H_