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+// © 2020 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
+
+#ifndef __MEASUNIT_IMPL_H__
+#define __MEASUNIT_IMPL_H__
+
+#include "unicode/utypes.h"
+
+#if !UCONFIG_NO_FORMATTING
+
+#include "unicode/measunit.h"
+#include "cmemory.h"
+#include "charstr.h"
+
+U_NAMESPACE_BEGIN
+
+
+static const char16_t kDefaultCurrency[] = u"XXX";
+static const char kDefaultCurrency8[] = "XXX";
+
+
+/**
+ * A struct representing a single unit (optional SI prefix and dimensionality).
+ */
+struct SingleUnitImpl : public UMemory {
+    /**
+     * Gets a single unit from the MeasureUnit. If there are multiple single units, sets an error
+     * code and returns the base dimensionless unit. Parses if necessary.
+     */
+    static SingleUnitImpl forMeasureUnit(const MeasureUnit& measureUnit, UErrorCode& status);
+
+    /** Transform this SingleUnitImpl into a MeasureUnit, simplifying if possible. */
+    MeasureUnit build(UErrorCode& status) const;
+
+    /**
+     * Compare this SingleUnitImpl to another SingleUnitImpl for the sake of
+     * sorting and coalescing.
+     *
+     * Takes the sign of dimensionality into account, but not the absolute
+     * value: per-meter is not considered the same as meter, but meter is
+     * considered the same as square-meter.
+     *
+     * The dimensionless unit generally does not get compared, but if it did, it
+     * would sort before other units by virtue of index being < 0 and
+     * dimensionality not being negative.
+     */
+    int32_t compareTo(const SingleUnitImpl& other) const {
+        if (dimensionality < 0 && other.dimensionality > 0) {
+            // Positive dimensions first
+            return 1;
+        }
+        if (dimensionality > 0 && other.dimensionality < 0) {
+            return -1;
+        }
+        if (index < other.index) {
+            return -1;
+        }
+        if (index > other.index) {
+            return 1;
+        }
+        if (siPrefix < other.siPrefix) {
+            return -1;
+        }
+        if (siPrefix > other.siPrefix) {
+            return 1;
+        }
+        return 0;
+    }
+
+    /**
+     * Return whether this SingleUnitImpl is compatible with another for the purpose of coalescing.
+     *
+     * Units with the same base unit and SI prefix should match, except that they must also have
+     * the same dimensionality sign, such that we don't merge numerator and denominator.
+     */
+    bool isCompatibleWith(const SingleUnitImpl& other) const {
+        return (compareTo(other) == 0);
+    }
+
+    /**
+     * Returns true if this unit is the "dimensionless base unit", as produced
+     * by the MeasureUnit() default constructor. (This does not include the
+     * likes of concentrations or angles.)
+     */
+    bool isDimensionless() const {
+        return index == -1;
+    }
+
+    /**
+     * Simple unit index, unique for every simple unit, -1 for the dimensionless
+     * unit. This is an index into a string list in measunit_extra.cpp.
+     *
+     * The default value is -1, meaning the dimensionless unit:
+     * isDimensionless() will return true, until index is changed.
+     */
+    int32_t index = -1;
+
+    /**
+     * SI prefix.
+     *
+     * This is ignored for the dimensionless unit.
+     */
+    UMeasureSIPrefix siPrefix = UMEASURE_SI_PREFIX_ONE;
+
+    /**
+     * Dimensionality.
+     *
+     * This is meaningless for the dimensionless unit.
+     */
+    int32_t dimensionality = 1;
+};
+
+
+/**
+ * Internal representation of measurement units. Capable of representing all complexities of units,
+ * including mixed and compound units.
+ */
+struct MeasureUnitImpl : public UMemory {
+    /** Extract the MeasureUnitImpl from a MeasureUnit. */
+    static inline const MeasureUnitImpl* get(const MeasureUnit& measureUnit) {
+        return measureUnit.fImpl;
+    }
+
+    /**
+     * Parse a unit identifier into a MeasureUnitImpl.
+     *
+     * @param identifier The unit identifier string.
+     * @param status Set if the identifier string is not valid.
+     * @return A newly parsed value object. Behaviour of this unit is
+     * unspecified if an error is returned via status.
+     */
+    static MeasureUnitImpl forIdentifier(StringPiece identifier, UErrorCode& status);
+
+    /**
+     * Extract the MeasureUnitImpl from a MeasureUnit, or parse if it is not present.
+     * 
+     * @param measureUnit The source MeasureUnit.
+     * @param memory A place to write the new MeasureUnitImpl if parsing is required.
+     * @param status Set if an error occurs.
+     * @return A reference to either measureUnit.fImpl or memory.
+     */
+    static const MeasureUnitImpl& forMeasureUnit(
+        const MeasureUnit& measureUnit, MeasureUnitImpl& memory, UErrorCode& status);
+
+    /**
+     * Extract the MeasureUnitImpl from a MeasureUnit, or parse if it is not present.
+     *
+     * @param measureUnit The source MeasureUnit.
+     * @param status Set if an error occurs.
+     * @return A value object, either newly parsed or copied from measureUnit.
+     */
+    static MeasureUnitImpl forMeasureUnitMaybeCopy(
+        const MeasureUnit& measureUnit, UErrorCode& status);
+
+    /**
+     * Used for currency units.
+     */
+    static inline MeasureUnitImpl forCurrencyCode(StringPiece currencyCode) {
+        MeasureUnitImpl result;
+        UErrorCode localStatus = U_ZERO_ERROR;
+        result.identifier.append(currencyCode, localStatus);
+        // localStatus is not expected to fail since currencyCode should be 3 chars long
+        return result;
+    }
+
+    /** Transform this MeasureUnitImpl into a MeasureUnit, simplifying if possible. */
+    MeasureUnit build(UErrorCode& status) &&;
+
+    /**
+     * Create a copy of this MeasureUnitImpl. Don't use copy constructor to make this explicit.
+     */
+    inline MeasureUnitImpl copy(UErrorCode& status) const {
+        MeasureUnitImpl result;
+        result.complexity = complexity;
+        result.units.appendAll(units, status);
+        result.identifier.append(identifier, status);
+        return result;
+    }
+
+    /** Mutates this MeasureUnitImpl to take the reciprocal. */
+    void takeReciprocal(UErrorCode& status);
+
+    /**
+     * Mutates this MeasureUnitImpl to append a single unit.
+     *
+     * @return true if a new item was added. If unit is the dimensionless unit,
+     * it is never added: the return value will always be false.
+     */
+    bool append(const SingleUnitImpl& singleUnit, UErrorCode& status);
+
+    /** The complexity, either SINGLE, COMPOUND, or MIXED. */
+    UMeasureUnitComplexity complexity = UMEASURE_UNIT_SINGLE;
+
+    /**
+     * The list of simple units. These may be summed or multiplied, based on the
+     * value of the complexity field.
+     *
+     * The "dimensionless" unit (SingleUnitImpl default constructor) must not be
+     * added to this list.
+     */
+    MaybeStackVector<SingleUnitImpl> units;
+
+    /**
+     * The full unit identifier.  Owned by the MeasureUnitImpl.  Empty if not computed.
+     */
+    CharString identifier;
+};
+
+
+U_NAMESPACE_END
+
+#endif /* #if !UCONFIG_NO_FORMATTING */
+#endif //__MEASUNIT_IMPL_H__