सुहृद्दुर्हृदौ मित्रामित्रयोः

Operational rule vidhi

The words सुहृत् and दुहृत् are irregularly formed बहुव्रीहि समास denoting 'a friend' and 'a foe' respectively.

Word by word (padaccheda)

Substitute / term सुहृद्दुर्हृदौ prathamā · dvivacanaRight context मित्रामित्रयोः saptamī · dvivacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

सुहृद्दुर्हृदौ मित्रामित्रयोः

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Commentary

The words सुहृत् and दुहृत् are irregularly formed बहुव्रीहि समास denoting 'a friend' and 'a foe' respectively.
The word सुहृत् is derived from सु + हृदय, the अय of हृदय being elided in the Bahuvrihi. It means one whose heart is favourably disposed towards another. So also दुर्हृत् is derived from दुर् + हृदय, he whose heart is evil.

Why do we say "when meaning a friend and a foe respectively"? Observe सुहृदयः कारुणिकः 'the kind-hearted, merciful'. दुर्हृदयश्चोरः "the evil hearted thief".

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.